fix: support older Gtk CssProvider API in frozen GTK client

cellar-gtk.py

@@ -145,7 +145,10 @@ def normalize_name(value: str | None) -> str:
 
 def _load_css() -> None:
     provider = Gtk.CssProvider()
-    provider.load_from_string(APP_CSS)
+    if hasattr(provider, "load_from_string"):
+        provider.load_from_string(APP_CSS)
+    else:
+        provider.load_from_data(APP_CSS.encode("utf-8"))
     Gtk.StyleContext.add_provider_for_display(
         Gdk.Display.get_default(),
         provider,