helium: follow the wallpaper theme via chromium GTK mode, drop its own frame

apps/helium.nix

@@ -1,9 +1,61 @@
 #./apps/helium.nix
 { inputs, pkgs, lib, config, ... }:
+let
+  upstream = inputs.helium-browser.packages.${pkgs.stdenv.hostPlatform.system}.default;
+
+  # Helium is ungoogled-chromium, so there is no userChrome.css to render a
+  # template into the way zen does. Its one themeable surface is chromium's
+  # GTK mode: with extensions.theme.system_theme = 1 (ui::SystemTheme::kGtk)
+  # the frame, tabstrip and omnibox come from the GTK3 theme, and chromium
+  # subscribes to notify::gtk-theme-name — so the gsettings flip at the end of
+  # theme-apply recolours a RUNNING helium, the same as nemo. Nothing
+  # per-wallpaper has to be generated for it.
+  #
+  # browser.custom_chrome_frame = false is chromium's "Use system title bar and
+  # borders". Hyprland draws no titlebar, so this only drops chromium's own
+  # frame, rounded corners and shadow margin — the window fills its tile.
+  #
+  # Both are profile prefs and chromium rewrites Preferences from memory on
+  # exit; no enterprise policy covers either, so there is no declarative way
+  # in. Patch the file just before launch instead: helium is then either not
+  # running, and the write sticks, or already running with the prefs applied
+  # and this process only forwards its arguments to that instance.
+  seedPrefs = pkgs.writeShellScript "helium-seed-prefs" ''
+    prefs="$HOME/.config/net.imput.helium/Default/Preferences"
+    [ -f "$prefs" ] || exit 0
+    ${pkgs.jq}/bin/jq -c '
+        .extensions.theme.system_theme = 1
+      | .browser.custom_chrome_frame = false
+    ' "$prefs" > "$prefs.fred" && mv "$prefs.fred" "$prefs" || rm -f "$prefs.fred"
+  '';
+in
 {
   config = lib.mkIf (lib.elem config.networking.hostName [ "FredOS-Gaming" "FredOS-Macbook" ]) {
     environment.systemPackages = [
-      inputs.helium-browser.packages.${pkgs.stdenv.hostPlatform.system}.default
+      (pkgs.symlinkJoin {
+        name = "helium-themed";
+        paths = [ upstream ];
+        nativeBuildInputs = [ pkgs.makeWrapper ];
+        # `|| true` is load-bearing: wrapProgram's wrapper runs under `bash -e`,
+        # so an unreadable or half-written Preferences would stop the browser
+        # from starting at all rather than just skipping the theming.
+        postBuild = ''
+          wrapProgram $out/bin/helium --run '${seedPrefs} || true'
+
+          # A broken filter here is invisible until the browser quietly never
+          # follows the theme, so patch a stub profile at build time and assert.
+          (
+            export HOME=$(mktemp -d)
+            mkdir -p "$HOME/.config/net.imput.helium/Default"
+            echo '{"browser":{"custom_chrome_frame":true}}' \
+              > "$HOME/.config/net.imput.helium/Default/Preferences"
+            ${seedPrefs}
+            ${pkgs.jq}/bin/jq -e '.extensions.theme.system_theme == 1
+                                  and .browser.custom_chrome_frame == false' \
+              "$HOME/.config/net.imput.helium/Default/Preferences" > /dev/null
+          )
+        '';
+      })
     ];
   };
 }

settings/theming.nix

@@ -7,9 +7,9 @@
 # every app that can reload without restarting.
 #
 # What follows a switch immediately: quickshell, hyprland, ghostty (D-Bus
-# reload-config), GTK3 apps such as nemo (each wallpaper installs its own
+# reload-config), GTK3 apps such as nemo and helium (each wallpaper installs
-# named GTK theme, switched over gsettings) and the folder icon tint. On next
+# its own named GTK theme, switched over gsettings) and the folder icon tint.
-# launch only: GTK4/libadwaita apps, btop, zen, spotify-player.
+# On next 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.