Add CSS to status area applet

Doesn't match other applets. Though toggle button behavior is probably
what we want?

applets/cosmic-applet-status-area/src/main.rs

@@ -14,6 +14,14 @@ fn main() {
     // XXX Implement DBus service somewhere other than applet?
     glib::MainContext::default().spawn_local(status_notifier_watcher::start());
 
+    let provider = gtk4::CssProvider::new();
+    provider.load_from_data(include_bytes!("style.css"));
+    gtk4::StyleContext::add_provider_for_display(
+        &gtk4::gdk::Display::default().expect("Could not connect to a display."),
+        &provider,
+        gtk4::STYLE_PROVIDER_PRIORITY_APPLICATION,
+    );
+
     let status_area = StatusArea::new();
     gtk4::Window::builder()
         .decorated(false)
@@ -21,6 +29,7 @@ fn main() {
         .resizable(false)
         .width_request(1)
         .height_request(1)
+        .css_classes(vec!["root_window".to_string()])
         .build()
         .show();