docs update

docs/examples/light_dark.py

@@ -0,0 +1,18 @@
+from textual.app import App
+from textual.widgets import Button
+
+
+class ButtonApp(App):
+    CSS = """
+    Button {
+        width: 100%;
+    }
+    """
+
+    def compose(self):
+        yield Button("Light", id="light")
+        yield Button("Dark", id="dark")
+
+    def handle_pressed(self, event):
+        self.dark = event.button.id == "dark"
+