Application: rely on Granite.StyleManager (#271)

Granite.StyleManager handles dark mode since 7.7.0

Author: danirabbit

src/Application.vala

@@ -17,15 +17,6 @@ public class Onboarding.App : Gtk.Application {
         base.startup ();
 
         Granite.init ();
-
-        var granite_settings = Granite.Settings.get_default ();
-        var gtk_settings = Gtk.Settings.get_default ();
-
-        gtk_settings.gtk_application_prefer_dark_theme = granite_settings.prefers_color_scheme == Granite.Settings.ColorScheme.DARK;
-
-        granite_settings.notify["prefers-color-scheme"].connect (() => {
-            gtk_settings.gtk_application_prefer_dark_theme = granite_settings.prefers_color_scheme == Granite.Settings.ColorScheme.DARK;
-        });
     }
 
     public override void activate () {