nixos/settings/theming.nix

# settings/theming.nix — runtime theme switching.
#
# stylix themes the system from ONE image at build time. This module builds a
# complete theme for EVERY image in walls/ instead, and swaps between them at
# runtime: the wallpaper picker in the quickshell bar writes a state file, and
# `theme-apply` drops that wallpaper's rendered config into place and nudges
# every app that can reload without restarting.
#
# What follows a switch immediately: quickshell, hyprland, vesktop, folder
# icons, and ghostty (reloaded over its D-Bus action). GTK apps only re-read
# their stylesheet at startup, so nemo etc. change on next launch; so do btop,
# zen and spotify-player.
#
# stylix stays the source of truth for build-time defaults and for every
# target not listed here — walls/wallpaper.png remains its image.
{ config, pkgs, lib, inputs, ... }:
let
  isDesktop = lib.elem config.networking.hostName [ "FredOS-Gaming" "FredOS-Macbook" ];
  homeDir = "/home/fred";
  stateLink = "${homeDir}/.local/state/quickshell-theme";
  zenChrome = ".zen/fraudek5.Default Profile/chrome";

  # ── Wallpapers ─────────────────────────────────────────────────────────
  # One palette derivation per image, not one for the set: generation is a
  # genetic search that takes seconds, so per-image derivations mean adding a
  # wallpaper only regenerates the new one. builtins.path hashes the image
  # content alone, so unrelated repo edits don't invalidate any of them.
  paletteGenerator =
    inputs.stylix.packages.${pkgs.stdenv.hostPlatform.system}.palette-generator;

  wallFiles = builtins.attrNames (lib.filterAttrs
    (n: t: t == "regular"
        && lib.any (e: lib.hasSuffix e (lib.toLower n)) [ ".png" ".jpg" ".jpeg" ])
    (builtins.readDir (inputs.self + "/walls")));

  # Store names allow a narrow charset, filenames don't. Keep wallpaper names
  # plain — a space survives, anything more exotic may not.
  wallStem = f: lib.head (lib.splitString "." f);
  wallSlug = f: lib.replaceStrings [ " " ] [ "-" ] (wallStem f);
  wallImage = f: builtins.path {
    name = "wall-" + lib.replaceStrings [ " " ] [ "-" ] f;
    path = inputs.self + "/walls/${f}";
  };
  wallPalette = f: pkgs.runCommand "palette-${wallSlug f}" { } ''
    mkdir -p $out
    ${paletteGenerator}/bin/palette-generator dark ${wallImage f} $out/palette.json
  '';

  # Import from derivation, the same way stylix reads its own generated
  # palette. Each wallpaper's palette is built during evaluation so its
  # colours can be rendered into config files here rather than at runtime.
  #
  # The `.override {}` is not decoration: bare mkSchemeAttrs output is not
  # callable as a template renderer, and calling it anyway returns a plain
  # attrset that stringifies to the template's path. stylix does the same
  # thing to build lib.stylix.colors.
  schemeOf = f: (config.stylix.base16.mkSchemeAttrs
    (builtins.fromJSON (builtins.readFile "${wallPalette f}/palette.json"))).override { };

  # ── Shared colour helpers ──────────────────────────────────────────────
  # Scaling all three channels by one factor is exactly what Qt.lighter()
  # does — hue and HSV saturation stay put, only value moves — so GTK lands
  # on the same surfaces as quickshell's Theme.base01/base02. GTK's own
  # shade() can't stand in: it multiplies saturation too.
  tint = s: f: let
    ch = k: toString (builtins.floor (builtins.fromJSON s."base00-rgb-${k}" * f));
  in "rgb(${ch "r"}, ${ch "g"}, ${ch "b"})";

  hexDigit = ch: {
    "0"=0;"1"=1;"2"=2;"3"=3;"4"=4;"5"=5;"6"=6;"7"=7;
    "8"=8;"9"=9;"a"=10;"b"=11;"c"=12;"d"=13;"e"=14;"f"=15;
  }.${ch};
  hexByte = s: hexDigit (builtins.substring 0 1 s) * 16
             + hexDigit (builtins.substring 1 1 s);
  hexToRgb = hex: let h = lib.toLower hex; in {
    r = hexByte (builtins.substring 0 2 h);
    g = hexByte (builtins.substring 2 2 h);
    b = hexByte (builtins.substring 4 2 h);
  };

  # papirus-folders palette → the front-face fill of each folder-<color>.svg,
  # read out of the icons themselves.
  #
  # NOT the Material Design values the names imply. Papirus paints the front
  # panel a good deal lighter than its own colour name: nordic's face is
  # 81a1c1, while 5e81ac is only the small back flap behind it. Matching a
  # teal accent against the nominal values is what made these folders come
  # out blue — the real darkcyan is 17x closer than nordic, the nominal one
  # wasn't.
  #
  # Every key here is a variant that actually ships (there is no "palegreen";
  # a name with no folder-<name>-*.svg files silently recolours nothing).
  pfPalette = {
    adwaita="93c0ea"; black="4f4f4f"; blue="5294e2"; bluegrey="607d8b";
    breeze="57b8ec"; brown="ae8e6c"; carmine="a30002"; cyan="00bcd4";
    darkcyan="45abb7"; deeporange="eb6637"; green="87b158"; grey="8e8e8e";
    indigo="5c6bc0"; magenta="ca71df"; nordic="81a1c1"; orange="ee923a";
    palebrown="d1bfae"; paleorange="eeca8f"; pink="f06292"; red="e25252";
    teal="16a085"; violet="7e57c2"; white="e4e4e4"; yaru="676767";
    yellow="f9bd30";
  };

  pfColorOf = s:
    let
      sq   = x: x * x;
      dist = a: b: sq (a.r - b.r) + sq (a.g - b.g) + sq (a.b - b.b);
      accent = hexToRgb s.base0D;
      ranked = map (n: { name = n; d = dist accent (hexToRgb pfPalette.${n}); })
                   (builtins.attrNames pfPalette);
    in (builtins.foldl' (best: x: if x.d < best.d then x else best)
                        (builtins.head ranked)
                        (builtins.tail ranked)).name;

  # Map Material You placeholders to the closest base16 slot. The mapping is
  # approximate (Material You has more semantic colours than base16), but
  # covers the placeholders used in our Zen userChrome and Vesktop quickCss
  # templates.
  stylixize = s: builtins.replaceStrings
    [
      "{{colors.primary.default.hex}}"
      "{{colors.primary_container.default.hex}}"
      "{{colors.secondary_container.default.hex}}"
      "{{colors.tertiary_container.default.hex}}"
      "{{colors.surface.default.hex}}"
      "{{colors.surface_container.default.hex}}"
      "{{colors.surface_container_low.default.hex}}"
      "{{colors.surface_container_high.default.hex}}"
      "{{colors.on_surface.default.hex}}"
      "{{colors.on_surface_variant.default.hex}}"
      "{{colors.outline.default.hex}}"
      "{{colors.outline_variant.default.hex}}"
    ]
    [
      "#${s.base0D}" # primary accent
      "#${s.base02}" # primary container
      "#${s.base0C}" # secondary container (cyan)
      "#${s.base0E}" # tertiary container (purple)
      "#${s.base00}" # surface (main bg)
      (tint s 1.7)   # surface container (urlbar, panels, dialogs)
      "#${s.base00}" # surface container low
      (tint s 2.4)   # surface container high (selected tab, focused urlbar)
      "#${s.base05}" # on surface (fg)
      "#${s.base04}" # on surface variant (muted fg)
      "#${s.base03}" # outline
      "#${s.base02}" # outline variant
    ];

  # ── Per-app renderers ──────────────────────────────────────────────────
  # GTK reuses stylix's own mustache template, so every @define-color stylix
  # would have written is still written — our overrides are appended after
  # it and therefore win.
  #
  # `template` is the mustache SOURCE, not a path to it: base16.nix only
  # treats the argument as a file when it is a path or a derivation, and
  # anything else — including the store-path string `builtins.path` hands
  # back — is written out verbatim as the template. That failure is silent
  # (you get a "stylesheet" containing one store path), which is what the
  # check in themeFor below exists to catch.
  gtkCssTemplate =
    builtins.readFile "${inputs.stylix}/modules/gtk/gtk.css.mustache";
  gtkCss = s: pkgs.concatText "gtk3.css" [
    (s { template = gtkCssTemplate; extension = ".css"; })
    (pkgs.writeText "gtk-extra.css" ''
      /* Surfaces: one hue, three steps — the same ladder quickshell's Theme
         uses. Stylix paints every raised surface with base01/base02, which
         come straight off the wallpaper and routinely land off-hue: base01
         is #3f3fab purple over a slate #1c1f26 desktop, which is why nemo's
         delete dialog was blue. */
      @define-color headerbar_bg_color ${tint s 1.7};
      @define-color sidebar_bg_color   ${tint s 1.7};
      @define-color card_bg_color      ${tint s 1.7};
      @define-color dialog_bg_color    ${tint s 1.7};
      @define-color popover_bg_color   ${tint s 1.7};
      @define-color scrollbar_outline_color ${tint s 2.4};

      headerbar { min-height: 0; padding: 0; margin: 0; }
      headerbar .title { font-size: 0; }
      window:backdrop {
        background-color: @window_bg_color;
        color: @window_fg_color;
      }
      window:backdrop headerbar {
        background-color: @headerbar_bg_color;
        color: @headerbar_fg_color;
      }
    '')
  ];

  # GTK4 reads gtk-theme-name and then ignores it, so the stylesheet has to be
  # pulled in by hand — this is what home-manager's own gtk4.theme option
  # writes, and taking over the file means taking this with it. It has to lead
  # the file: everything after an @import overrides it, which is exactly the
  # order the colours want.
  gtk4Css = s: pkgs.concatText "gtk4.css" [
    (pkgs.writeText "adw-gtk3-import.css" ''
      @import url("file://${pkgs.adw-gtk3}/share/themes/adw-gtk3/gtk-4.0/gtk.css");
    '')
    (gtkCss s)
  ];

  # Ghostty theme files are plain config syntax, so this is included from the
  # main config with `config-file` rather than installed as a named theme.
  ghosttyConf = s: pkgs.writeText "ghostty-theme.conf" (with s.withHashtag; ''
    background = ${base00}
    foreground = ${base05}
    cursor-color = ${base05}
    selection-background = ${base02}
    selection-foreground = ${base05}
    palette = 0=${base00}
    palette = 1=${base08}
    palette = 2=${base0B}
    palette = 3=${base0A}
    palette = 4=${base0D}
    palette = 5=${base0E}
    palette = 6=${base0C}
    palette = 7=${base05}
    palette = 8=${base03}
    palette = 9=${base08}
    palette = 10=${base0B}
    palette = 11=${base0A}
    palette = 12=${base0D}
    palette = 13=${base0E}
    palette = 14=${base0C}
    palette = 15=${base07}
  '');

  btopTheme = s: pkgs.writeText "btop.theme" (with s.withHashtag; ''
    #Generated by settings/theming.nix
    theme[main_bg]="${base00}"
    theme[main_fg]="${base05}"
    theme[title]="${base05}"
    theme[hi_fg]="${base0D}"
    theme[selected_bg]="${base03}"
    theme[selected_fg]="${base0D}"
    theme[inactive_fg]="${base04}"
    theme[graph_text]="${base06}"
    theme[meter_bg]="${base03}"
    theme[proc_misc]="${base06}"
    theme[cpu_box]="${base0E}"
    theme[mem_box]="${base0B}"
    theme[net_box]="${base0C}"
    theme[proc_box]="${base0D}"
    theme[div_line]="${base01}"
    theme[temp_start]="${base0B}"
    theme[temp_mid]="${base0A}"
    theme[temp_end]="${base08}"
    theme[cpu_start]="${base0B}"
    theme[cpu_mid]="${base0A}"
    theme[cpu_end]="${base08}"
    theme[free_start]="${base0A}"
    theme[free_mid]="${base0B}"
    theme[free_end]="${base0B}"
    theme[cached_start]="${base0C}"
    theme[cached_mid]="${base0C}"
    theme[cached_end]="${base0A}"
    theme[available_start]="${base08}"
    theme[available_mid]="${base0A}"
    theme[available_end]="${base0B}"
    theme[used_start]="${base0A}"
    theme[used_mid]="${base09}"
    theme[used_end]="${base08}"
    theme[download_start]="${base0B}"
    theme[download_mid]="${base0A}"
    theme[download_end]="${base08}"
    theme[upload_start]="${base0B}"
    theme[upload_mid]="${base0A}"
    theme[upload_end]="${base08}"
    theme[process_start]="${base0B}"
    theme[process_mid]="${base0A}"
    theme[process_end]="${base08}"
  '');

  # Lua, fed to `hyprctl eval`. NOT `hyprctl keyword`, which answers
  #   "keyword can't work with non-legacy parsers. Use eval."
  # under the Lua config this repo uses (settings/hyprland.nix sets
  # configType = "lua"). eval runs in the live VM with isDynamicParse() true,
  # so hl.config schedules the refresh that actually repaints the borders.
  #
  # Keys are dot-joined here, matching how the Lua config manager flattens
  # nested tables — the same shape settings/hyprland.nix already declares,
  # which stays the build-time fallback until theme-apply first runs.
  #
  # One line, not pretty-printed: hyprctl joins its argv into the request
  # string, so keeping the payload free of newlines keeps it intact.
  hyprLua = s: pkgs.writeText "hypr.lua" ("hl.config({" + lib.concatStringsSep "," [
    ''general={["col.active_border"]="rgb(${s.base04})",["col.inactive_border"]="rgb(${s.base03})"}''
    ''decoration={shadow={color="rgba(${s.base00}99)"}}''
    (''group={["col.border_active"]="rgb(${s.base04})",''
     + ''["col.border_inactive"]="rgb(${s.base03})",''
     + ''["col.border_locked_active"]="rgb(${s.base05})",''
     + ''groupbar={text_color="rgb(${s.base05})",''
     + ''["col.active"]="rgb(${s.base04})",["col.inactive"]="rgb(${s.base03})"}}'')
    ''misc={background_color="rgb(${s.base00})"}''
  ] + "})");

  # A featherweight icon theme that inherits Papirus-Dark and overrides only
  # the folder faces, rather than a recoloured copy of the whole 200MB tree
  # per wallpaper. Papirus-Dark delegates its size dirs to ../Papirus by
  # symlink, so the sources are resolved through the real files.
  iconTheme = s: let color = pfColorOf s; in
    pkgs.runCommand "papirus-fred-${color}" { } ''
      sizes="22x22 24x24 32x32 48x48 64x64"
      mkdir -p $out
      {
        echo "[Icon Theme]"
        echo "Name=Papirus-Fred"
        echo "Comment=Papirus-Dark with ${color} folders"
        echo "Inherits=Papirus-Dark,Papirus,hicolor"
        printf 'Directories='
        first=1
        for size in $sizes; do
          [ $first -eq 1 ] || printf ','
          printf '%s/places' "$size"
          first=0
        done
        echo
        for size in $sizes; do
          echo
          echo "[$size/places]"
          echo "Size=''${size%%x*}"
          echo "Context=Places"
          echo "Type=Fixed"
        done
      } > $out/index.theme

      for size in $sizes; do
        src=${pkgs.papirus-icon-theme}/share/icons/Papirus-Dark/$size/places
        [ -d "$src" ] || continue
        mkdir -p $out/$size/places
        for f in "$src"/folder-${color}*.svg "$src"/user-${color}*.svg; do
          [ -f "$f" ] || continue
          fname=$(basename "$f")
          # folder-darkcyan-cloud.svg -> folder-cloud.svg; the generic name is
          # what nemo actually asks for.
          ln -sf "$(readlink -f "$f")" "$out/$size/places/''${fname/-${color}/}"
        done
      done
    '';

  # ── One theme directory per wallpaper ──────────────────────────────────
  themeFor = f: let s = schemeOf f; in pkgs.runCommand "theme-${wallSlug f}" { } ''
    mkdir -p $out
    cp ${wallPalette f}/palette.json $out/palette.json
    cp ${gtkCss s}          $out/gtk3.css
    cp ${gtk4Css s}         $out/gtk4.css
    cp ${ghosttyConf s}     $out/ghostty.conf
    cp ${btopTheme s}       $out/btop.theme
    cp ${hyprLua s}         $out/hypr.lua
    cp ${pkgs.writeText "vesktop.css" (stylixize s (builtins.readFile "${inputs.self}/templates/vesktop-quickCss.css"))} $out/vesktop.css
    cp ${pkgs.writeText "zen.css"     (stylixize s (builtins.readFile "${inputs.self}/templates/zen-userChrome.css"))}   $out/zen.css
    ln -s ${iconTheme s} $out/icons

    # Cheap smoke test: every one of these is generated by a different
    # mechanism, and a silent miss (a template that didn't render, a scheme
    # attr that came back empty) is invisible until the desktop is ugly.
    check() {
      grep -q "$2" "$out/$1" || {
        echo "theming: $out/$1 is missing /$2/ — renderer broke" >&2
        exit 1
      }
    }
    check gtk3.css    '@define-color window_bg_color'
    check gtk4.css    'adw-gtk3/gtk-4.0/gtk.css'
    check ghostty.conf '^background = #'
    check hypr.lua    'col.active_border"]="rgb('
    check vesktop.css '^  --bg-4: #'
  '';

  walls = map (f: {
    name = wallStem f;
    image = "${wallImage f}";
    theme = "${themeFor f}";
  }) wallFiles;

  # Falls back to walls/wallpaper.png — the one stylix themes the build with
  # — so a missing state file lands on the build-time scheme.
  defaultWall = if lib.elem "wallpaper" (map wallStem wallFiles)
                then "wallpaper"
                else wallStem (lib.head wallFiles);

  # ── The switcher ───────────────────────────────────────────────────────
  # Copies rather than symlinks into ~/.config: GTK and vesktop reload by
  # watching those files, and a symlink whose target changed underneath
  # doesn't fire a file monitor.
  themeApply = pkgs.writeShellScript "theme-apply" ''
    set -u
    theme="$1"
    [ -d "$theme" ] || exit 0

    # Everything from here lands in one log, truncated per run. NOT the
    # journal: quickshell starts this with execDetached, which drops the
    # child's stdio, so anything written to stderr goes nowhere. This file is
    # the only record of what a switch actually did.
    #   cat ~/.local/state/theme-apply.log
    log="$HOME/.local/state/theme-apply.log"
    mkdir -p "$(dirname "$log")"
    exec >"$log" 2>&1
    echo "theme-apply $(date -Is): $theme"

    ln -sfn "$theme" ${lib.escapeShellArg stateLink}

    # Only the files an app insists on finding at a fixed path get copied.
    # Ghostty and btop are pointed straight at the state symlink instead
    # (see the home-manager block below) — one less write to go wrong, and no
    # way for a stale copy to survive a switch.
    install -Dm644 "$theme/gtk3.css"    "$HOME/.config/gtk-3.0/gtk.css"
    install -Dm644 "$theme/gtk4.css"    "$HOME/.config/gtk-4.0/gtk.css"
    install -Dm644 "$theme/vesktop.css" "$HOME/.config/vesktop/settings/quickCss.css"
    install -Dm644 "$theme/zen.css"     "$HOME/${zenChrome}/userChrome.css"

    # Icon theme: replace the directory contents (cheap — the tree is
    # symlinks into the papirus store path) so GTK's icon-theme monitor sees
    # a change and rescans.
    icons="$HOME/.local/share/icons/Papirus-Fred"
    rm -rf "$icons"
    mkdir -p "$icons"
    cp -r "$theme"/icons/. "$icons"/
    chmod -R u+w "$icons"

    # Hyprland takes its colours live. `hyprctl eval`, not `hyprctl keyword`:
    # keyword is a legacy-parser command and refuses outright under the Lua
    # config. eval answers "ok", or a string naming every key it rejected —
    # hl.config carries on past a bad key, so the rest still land.
    reply=$(${pkgs.hyprland}/bin/hyprctl eval "$(cat "$theme/hypr.lua")" 2>&1)
    [ "$reply" = "ok" ] || echo "hyprctl eval -> $reply"

    # Ghostty holds its config in one process (gtk-single-instance), so a new
    # window inherits whatever the daemon parsed at startup — reopening a
    # terminal is NOT enough to pick up a new theme.
    #
    # It is a GTK application, so it publishes its actions on the session bus;
    # `reload-config` is the same action the reload_config keybind fires. Going
    # through D-Bus works with no window open, needs no window focused, and
    # doesn't depend on synthesising a keypress into the right surface.
    if ! ${pkgs.glib.bin}/bin/gdbus call --session \
        --dest com.mitchellh.ghostty \
        --object-path /com/mitchellh/ghostty \
        --method org.gtk.Actions.Activate \
        reload-config '[]' '{}' >/dev/null 2>&1; then
      # No instance on the bus, or the action is gone. Restarting the daemon
      # fixes it but takes every open terminal with it, so that is only on the
      # table when there is nothing to lose. The class test has to cover the
      # --class overrides from settings/hyprland.nix (dev.fred.scratchpad,
      # dev.fred.spotify) — those windows are the same process.
      if ${pkgs.hyprland}/bin/hyprctl -j clients 2>/dev/null \
          | ${pkgs.jq}/bin/jq -e 'any(.[].class; test("ghostty|^dev\\.fred\\."))' >/dev/null 2>&1; then
        echo "ghostty reload-config failed; open terminals keep the old theme"
      else
        ${pkgs.systemd}/bin/systemctl --user restart ghostty-daemon.service 2>/dev/null || true
      fi
    fi

    # GTK reads ~/.config/gtk-{3,4}.0/gtk.css once at startup and does not
    # watch it, so running apps only re-read on a theme-name change. Toggling
    # the gsettings key away and back is the one nudge that reaches them, and
    # only lands if something is bridging gsettings to GTK (the portal). Nemo
    # picks the new colours up on next launch regardless.
    gsettings=${pkgs.glib.bin}/bin/gsettings
    iface=org.gnome.desktop.interface
    if cur=$($gsettings get $iface gtk-theme 2>/dev/null); then
      cur=''${cur//\'/}
      # The detour has to be a DIFFERENT name or gsettings writes the same
      # value, emits no change signal, and nothing reloads.
      away=Adwaita
      [ "$cur" = "$away" ] && away=Adwaita-dark
      $gsettings set $iface gtk-theme "$away" 2>/dev/null || true
      $gsettings set $iface gtk-theme "$cur" 2>/dev/null || true
    fi
  '';
in
{
  options.fred.theming = {
    walls = lib.mkOption {
      internal = true;
      type = lib.types.listOf (lib.types.attrsOf lib.types.str);
      description = "Every wallpaper in walls/, with its image and theme directory.";
    };
    defaultWall = lib.mkOption { internal = true; type = lib.types.str; };
    apply = lib.mkOption { internal = true; type = lib.types.path; };
  };

  config = lib.mkIf isDesktop {
    fred.theming = {
      inherit walls defaultWall;
      apply = themeApply;
    };

    # stylix ships fish as BOTH a NixOS and a home-manager target, and this is
    # the NixOS half — programs.fish.interactiveShellInit in /etc/fish. Turning
    # off only the home-manager one left this still sourcing base16-fish, which
    # printf's OSC 4 / OSC 10 / OSC 11 at every shell start and repaints the
    # build-time palette over whatever ghostty just reloaded. Both halves have
    # to go or the terminal never changes colour.
    stylix.targets.fish.enable = false;

    home-manager.users.fred = { config, lib, pkgs, osConfig, ... }: {
      # These four targets write build-time files that theme-apply replaces
      # at runtime; leaving them on means stylix and the switcher fight over
      # the same paths on every rebuild.
      stylix.targets.gtk.enable = false;
      stylix.targets.ghostty.enable = false;
      stylix.targets.btop.enable = false;

      # The one that made ghostty look unthemeable. stylix's fish target
      # sources base16-fish, which is a base16-SHELL script: at every shell
      # start it printf's OSC 4 (palette 0-15) and OSC 10/11 (fg/bg) with the
      # BUILD-TIME colours, overwriting whatever ghostty just loaded. So a
      # wallpaper switch reloaded ghostty correctly and fish immediately
      # painted the old scheme back over it — and opening a new window ran
      # fish again, which is why reopening never helped either. Only
      # selection-background, which base16-fish does not touch, got through.
      #
      # Off, fish leaves the palette alone and inherits the terminal's, so
      # every ANSI-coloured program in a ghostty window follows a switch live.
      stylix.targets.fish.enable = false;

      # Everything stylix's gtk target did apart from writing gtk.css.
      gtk.theme = { package = pkgs.adw-gtk3; name = "adw-gtk3"; };
      # Explicitly null, not merely unset: home-manager defaults gtk4.theme to
      # gtk.theme and then writes gtk-4.0/gtk.css to @import the theme, which
      # is a file theme-apply owns. gtk4Css reproduces that import itself.
      gtk.gtk4.theme = null;
      gtk.font = {
        inherit (osConfig.stylix.fonts.sansSerif) package name;
        size = osConfig.stylix.fonts.sizes.applications;
      };
      # Written into ~/.local/share/icons by theme-apply; the package is here
      # so Papirus-Dark — which Papirus-Fred inherits — is installed.
      gtk.iconTheme = {
        package = pkgs.papirus-icon-theme;
        name = "Papirus-Fred";
      };

      # Both read straight through the state symlink, so a switch is nothing
      # but the `ln -sfn` in theme-apply — no copy to fail, nothing stale.
      #
      # The leading `?` marks the include optional: before the first
      # theme-apply of a fresh account the link does not exist yet, and
      # without it ghostty treats that as a config error. Includes are also
      # applied AFTER the file that names them, so these colours win over
      # anything home-manager writes above.
      programs.ghostty.settings.config-file = "?${stateLink}/ghostty.conf";
      # Manual escape hatch; the switcher itself reloads over D-Bus.
      programs.ghostty.settings.keybind = [ "ctrl+shift+alt+f12=reload_config" ];
      programs.btop.settings.color_theme = "${stateLink}/btop.theme";
    };
  };
}