enzo: serve portal color-scheme from gtk backend so web content goes dark

LibreWolf chrome was dark (via GTK theme) but web content stayed light. Root
cause is the portal, not the browser: the niri module routes the Settings
interface to xdg-desktop-portal-gnome, whose color-scheme depends on
gnome-settings-daemon. Under bare niri (no GNOME session) gsd is absent, so the
portal reported "no preference" and content's prefers-color-scheme resolved
light.

Pin org.freedesktop.impl.portal.Settings to the gtk backend, which reads the
org.gnome.desktop.interface color-scheme dconf key directly (no gsd), and add
xdg-desktop-portal-gtk. The key merges into the niri module's
xdg.portal.config.niri. Now the portal reports prefer-dark and content follows.

Assisted-by: Claude:claude-opus-4-8

1 files changed, 16 insertions, 5 deletions

diff --git a/machines/enzo/desktop.nix b/machines/enzo/desktop.nix
index 9d543fb..4d1c29b 100644
--- a/machines/enzo/desktop.nix
+++ b/machines/enzo/desktop.nix
@@ -8,17 +8,28 @@
   programs.niri.enable = true;
   programs.dconf.enable = true;
 
-  # Tell the desktop it's in dark mode. niri has no settings daemon; apps learn
-  # light/dark from xdg-desktop-portal's Settings interface, whose gnome/gtk
-  # backends read this dconf key. Setting it here makes the portal report
-  # "prefer-dark", so Firefox/LibreWolf (Auto theme + web content) and GTK apps
-  # all go dark — no per-app theme config needed.
+  # Tell the desktop it's in dark mode. Apps read light/dark from
+  # xdg-desktop-portal's Settings interface (org.freedesktop.appearance
+  # color-scheme). This dconf default is the value the portal serves.
   programs.dconf.profiles.user.databases = [
     {
       settings."org/gnome/desktop/interface".color-scheme = "prefer-dark";
     }
   ];
 
+  # Serve color-scheme from the *gtk* portal backend, not gnome. The niri module
+  # routes Settings to xdg-desktop-portal-gnome (its default), but that backend
+  # relies on gnome-settings-daemon — absent under bare niri — so it reports
+  # "no preference" and web content (LibreWolf's prefers-color-scheme) stays
+  # light even though GTK chrome follows the dark GTK theme. xdg-desktop-portal-
+  # gtk reads the dconf color-scheme above directly, so pinning Settings to it
+  # makes the portal actually report dark and content follows. This key merges
+  # into the niri module's xdg.portal.config.niri.
+  xdg.portal = {
+    extraPortals = [ pkgs.xdg-desktop-portal-gtk ];
+    config.niri."org.freedesktop.impl.portal.Settings" = "gtk";
+  };
+
   services.greetd = {
     enable = true;
     settings.default_session = {