Add wip test for user over widget css, use type alias

sandbox/darren/just_a_box.py

@@ -0,0 +1,12 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Static
+
+
+class JustABox(App):
+    def compose(self) -> ComposeResult:
+        yield Static("Hello, World!", id="box")
+
+
+if __name__ == "__main__":
+    app = JustABox(css_path="just_a_box.css", watch_css=True)
+    app.run()