flatpak overrides: bake GTK_THEME=adw-gtk3-dark + force-dark color scheme

Operator 2026-05-10: "Nautilus STILL has olde GTK/CSS/Decorations/
theming" -- the dconf system-db override (mios.git@8f29504) set
host gtk-theme=adw-gtk3-dark + color-scheme=prefer-dark, but
flatpaks don't see host themes by default. Their bundled Adwaita
runtime falls back to its built-in light theme.

Two coupled fixes:

1. /var/lib/flatpak/overrides/<app>: add GTK_THEME=adw-gtk3-dark +
   ADW_DEBUG_COLOR_SCHEME=force-dark. Combined with the
   org.gtk.Gtk3theme.adw-gtk3-dark flatpak extension (added to
   mios-bootstrap/mios.toml [desktop].flatpaks), each sandboxed
   GTK app picks up the dark Adwaita theme natively -- same
   look as system-side gnome-software etc.

2. ADW_DEBUG_COLOR_SCHEME=force-dark forces libadwaita apps
   (which ignore GTK_THEME because they use their own stylesheet)
   into dark mode at the libadwaita level.

Verified live 2026-05-10: nautilus relaunch with new env -- the
GLib G_IS_OBJECT assertion crashes vanished, replaced by a
benign "Theme parsing error: 'font-feature-settings' is not a
valid property name" (CSS warning, not fatal). Confirms the
adw-gtk3-dark CSS is actually being parsed + applied.

Author: Kabuki94

var/lib/flatpak/overrides/com.github.tchx84.Flatseal

@@ -4,3 +4,5 @@ LIBGL_ALWAYS_SOFTWARE=1
 GALLIUM_DRIVER=llvmpipe
 LIBGL_KOPPER_DISABLE=1
 WEBKIT_DISABLE_COMPOSITING_MODE=1
+GTK_THEME=adw-gtk3-dark
+ADW_DEBUG_COLOR_SCHEME=force-dark

var/lib/flatpak/overrides/com.mattjakeman.ExtensionManager

@@ -4,3 +4,5 @@ LIBGL_ALWAYS_SOFTWARE=1
 GALLIUM_DRIVER=llvmpipe
 LIBGL_KOPPER_DISABLE=1
 WEBKIT_DISABLE_COMPOSITING_MODE=1
+GTK_THEME=adw-gtk3-dark
+ADW_DEBUG_COLOR_SCHEME=force-dark

var/lib/flatpak/overrides/com.vscodium.codium

@@ -4,3 +4,5 @@ LIBGL_ALWAYS_SOFTWARE=1
 GALLIUM_DRIVER=llvmpipe
 LIBGL_KOPPER_DISABLE=1
 WEBKIT_DISABLE_COMPOSITING_MODE=1
+GTK_THEME=adw-gtk3-dark
+ADW_DEBUG_COLOR_SCHEME=force-dark

var/lib/flatpak/overrides/org.gnome.Epiphany

@@ -4,3 +4,5 @@ LIBGL_ALWAYS_SOFTWARE=1
 GALLIUM_DRIVER=llvmpipe
 LIBGL_KOPPER_DISABLE=1
 WEBKIT_DISABLE_COMPOSITING_MODE=1
+GTK_THEME=adw-gtk3-dark
+ADW_DEBUG_COLOR_SCHEME=force-dark

var/lib/flatpak/overrides/org.gnome.Nautilus

@@ -4,3 +4,5 @@ LIBGL_ALWAYS_SOFTWARE=1
 GALLIUM_DRIVER=llvmpipe
 LIBGL_KOPPER_DISABLE=1
 WEBKIT_DISABLE_COMPOSITING_MODE=1
+GTK_THEME=adw-gtk3-dark
+ADW_DEBUG_COLOR_SCHEME=force-dark

var/lib/flatpak/overrides/org.gnome.Software

@@ -4,3 +4,5 @@ LIBGL_ALWAYS_SOFTWARE=1
 GALLIUM_DRIVER=llvmpipe
 LIBGL_KOPPER_DISABLE=1
 WEBKIT_DISABLE_COMPOSITING_MODE=1
+GTK_THEME=adw-gtk3-dark
+ADW_DEBUG_COLOR_SCHEME=force-dark