main: Consider color-scheme setting for default stylesheet

Our own preference - equivalent to AdwStyleManager's color-scheme
property - is expressed via a new session mode property.

The default mode uses 'prefer-dark', gnome-classic is expected
to use 'force-light'.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5545

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2324>

js/ui/sessionMode.js

@@ -22,7 +22,8 @@ if (Config.HAVE_NETWORKMANAGER)
 const _modes = {
     'restrictive': {
         parentMode: null,
-        stylesheetName: 'gnome-shell-dark.css',
+        stylesheetName: 'gnome-shell.css',
+        colorScheme: 'prefer-dark',
         themeResourceName: 'gnome-shell-theme.gresource',
         hasOverview: false,
         showCalendarEvents: false,