theming: live-retheme GTK3 apps via a named theme per wallpaper

GTK3 parses ~/.config/gtk-3.0/gtk.css once per process behind a static
guard and never re-reads it, and it outranks the theme at PRIORITY_USER —
so colours there pin an app to whatever was live when it started. Install
a Fred-<wallpaper> theme instead and switch gtk-theme over gsettings,
which xdg-desktop-portal-gtk forwards to running apps.

One name per wallpaper because gtk_css_provider_get_named memoises
providers in a static hash table keyed by name.

GTK4/libadwaita ignores gtk-theme-name, so it keeps the user CSS and
still needs a restart.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

settings/theming.nix

@@ -6,10 +6,10 @@
 # `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
+# What follows a switch immediately: quickshell, hyprland, ghostty (D-Bus
-# icons, and ghostty (reloaded over its D-Bus action). GTK apps only re-read
+# reload-config), GTK3 apps such as nemo (each wallpaper installs its own
-# their stylesheet at startup, so nemo etc. change on next launch; so do btop,
+# named GTK theme, switched over gsettings) and the folder icon tint. On next
-# zen and spotify-player.
+# launch only: GTK4/libadwaita apps, btop, zen, 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.
@@ -337,11 +337,60 @@ let
       done
     '';
 
+  # A real, installed GTK3 theme per wallpaper — the only way a RUNNING GTK3
+  # app ever changes colour.
+  #
+  # ~/.config/gtk-3.0/gtk.css cannot do it. gtksettings.c loads that file
+  # behind `if (G_UNLIKELY (!css_provider))` on a static, so it is parsed once
+  # per process and never re-read; and it sits at PRIORITY_USER, above the
+  # theme, so leaving a stale copy there would also outrank whatever the theme
+  # supplies. The named-theme provider, by contrast, is reloaded whenever
+  # gtk-theme-name changes — that signal reaches apps here through
+  # xdg-desktop-portal-gtk.
+  #
+  # Hence one theme NAME per wallpaper rather than a single "Fred" that gets
+  # rewritten: gtk_css_provider_get_named memoises providers in a static hash
+  # table keyed by name, so reusing the name would hand back the CSS parsed
+  # the first time. Distinct names also make switching back to an earlier
+  # wallpaper a cache hit with the right colours.
+  gtkThemeName = f: "Fred-${wallSlug f}";
+  gtkTheme = f: s: pkgs.runCommand "gtk-theme-${wallSlug f}" { } ''
+    mkdir -p $out/gtk-3.0
+    # GTK resolves the theme's relative url(assets/...) against the theme dir,
+    # not the imported file, so the artwork has to be reachable from here.
+    ln -s ${pkgs.adw-gtk3}/share/themes/adw-gtk3/gtk-3.0/assets $out/gtk-3.0/assets
+
+    # Both variants: gtk-application-prefer-dark-theme is on, so GTK asks for
+    # gtk-dark.css, and _gtk_css_provider_load_named falls back to gtk.css
+    # only if the variant is missing entirely.
+    for v in gtk.css gtk-dark.css; do
+      {
+        echo "@import url(\"file://${pkgs.adw-gtk3}/share/themes/adw-gtk3/gtk-3.0/$v\");"
+        cat ${gtkCss s}
+      } > $out/gtk-3.0/$v
+    done
+
+    cat > $out/index.theme <<'EOF'
+    [Desktop Entry]
+    Type=X-GNOME-Metatheme
+    Name=${gtkThemeName f}
+    Comment=adw-gtk3 tinted to ${wallStem f}
+    Encoding=UTF-8
+
+    [X-GNOME-Metatheme]
+    GtkTheme=${gtkThemeName f}
+    EOF
+
+    grep -q '@import url' $out/gtk-3.0/gtk-dark.css
+    grep -q '@define-color window_bg_color' $out/gtk-3.0/gtk-dark.css
+  '';
+
   # ── One theme directory per wallpaper ──────────────────────────────────
   themeFor = f: let s = schemeOf f; in pkgs.runCommand "theme-${wallSlug f}" { } ''
     mkdir -p $out
     cp ${wallPalette f}/palette.json $out/palette.json
     cp ${gtkCss s}          $out/gtk3.css
+    echo -n "${gtkThemeName f}" > $out/gtk-theme-name
     cp ${gtk4Css s}         $out/gtk4.css
     cp ${ghosttyConf s}     $out/ghostty.conf
     cp ${btopTheme s}       $out/btop.theme
@@ -403,7 +452,14 @@ let
     # Ghostty and btop are pointed straight at the state symlink instead
     # (see the home-manager block below) — one less write to go wrong, and no
     # way for a stale copy to survive a switch.
-    install -Dm644 "$theme/gtk3.css"    "$HOME/.config/gtk-3.0/gtk.css"
+    # NOTE: no ~/.config/gtk-3.0/gtk.css. GTK3's colours come from the
+    # installed Fred-<wallpaper> theme instead (see gtkTheme above); a copy
+    # here would sit at PRIORITY_USER, outranking the theme, and GTK3 parses
+    # it once per process — so it would pin every GTK3 app to whichever
+    # wallpaper was live when the app started.
+    #
+    # GTK4 still needs it: libadwaita ignores gtk-theme-name, so user CSS is
+    # the only way in, and those apps do change only on restart.
     install -Dm644 "$theme/gtk4.css"    "$HOME/.config/gtk-4.0/gtk.css"
     # Vesktop gets the same CSS twice, on purpose.
     #
@@ -463,22 +519,13 @@ let
       fi
     fi
 
-    # GTK reads ~/.config/gtk-{3,4}.0/gtk.css once at startup and does not
+    # Point GTK3 at this wallpaper's theme. Every wallpaper has its own theme
-    # watch it, so running apps only re-read on a theme-name change. Toggling
+    # name, so this is always a real change: gsettings emits, the portal
-    # the gsettings key away and back is the one nudge that reaches them, and
+    # forwards it, and running GTK3 apps reload the named theme — no restart,
-    # only lands if something is bridging gsettings to GTK (the portal). Nemo
+    # no away-and-back dance.
-    # picks the new colours up on next launch regardless.
+    gtktheme=$(cat "$theme/gtk-theme-name")
-    gsettings=${pkgs.glib.bin}/bin/gsettings
+    ${pkgs.glib.bin}/bin/gsettings set org.gnome.desktop.interface gtk-theme "$gtktheme" \
-    iface=org.gnome.desktop.interface
+      || echo "gsettings set gtk-theme $gtktheme failed"
-    if cur=$($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
 {
@@ -527,8 +574,18 @@ in
       # 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.
+      # Every wallpaper's GTK3 theme, installed up front. theme-apply only
-      gtk.theme = { package = pkgs.adw-gtk3; name = "adw-gtk3"; };
+      # flips gtk-theme between them, so they all have to exist on disk —
+      # GTK silently falls back to Adwaita for a name it cannot find.
+      home.file = lib.listToAttrs (map (f: lib.nameValuePair
+        ".local/share/themes/${gtkThemeName f}"
+        { source = gtkTheme f (schemeOf f); })
+        wallFiles);
+
+      # Everything stylix's gtk target did apart from writing gtk.css. The
+      # name here is only the pre-first-switch fallback and what lands in
+      # settings.ini; the live value comes from gsettings via the portal.
+      gtk.theme = { package = pkgs.adw-gtk3; name = gtkThemeName (lib.head wallFiles); };
       # 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.