fix styles examples, box model docs

docs/examples/guide/styles/screen.py

@@ -0,0 +1,12 @@
+from textual.app import App
+
+
+class ScreenApp(App):
+    def on_mount(self) -> None:
+        self.screen.styles.background = "darkblue"
+        self.screen.styles.border = ("heavy", "white")
+
+
+app = ScreenApp()
+if __name__ == "__main__":
+    app.run()