feat(helium): switch to the GTK theme with KWin title buttons

Helium now takes its colors from Breeze GTK instead of a generated
theme extension, which reset the GTK theme on every launch.

- Import window_decorations.css from gtk.css so Helium, which doesn't
  load kde-gtk-config's GTK module, draws the KWin title buttons
  instead of Breeze GTK's round ones.
- Add chromium.css to give the compact layout's address bar and
  button segment the title bar color, so the tab strip is one color.
- Disable WaylandWpColorManagerV1 so KWin doesn't lift Helium's dark
  grays (40 -> 44) and the strip matches KDE title bars.
- Drop the Helium theme from build.py and the helium package.

kde/helium/README.md

@@ -1,11 +1,14 @@
 # Helium
 
-Browser flags and the generated Darwin Breeze theme. From the repository root:
+Browser flags for hardware video decoding on Wayland and for colors that
+match KDE apps. From the repository root:
 
 ```sh
 stow -d kde -t "$HOME" helium
 ```
 
-The theme files are included, so the KDE `theme` package does not need to be
-stowed to use Helium's theme. To regenerate its colors from the shared color
-scheme, run `python3 kde/theme/build.py` and restart Helium.
+Helium takes its colors and window buttons from GTK. In Settings →
+Appearance, set Theme to GTK and turn off "Use system title bar and borders".
+With the KDE `theme` package applied, GTK follows the Darwin Breeze colors and
+the window buttons match KWin's; see
+[Darwin Breeze → Helium notes](../theme/README.md#helium-notes).