Make TextArea respond to app.theme changes.

src/textual/widgets/_text_area.py

@@ -1049,12 +1049,17 @@ def gutter_width(self) -> int:
         )
         return gutter_width
 
+    def _retheme(self) -> None:
+        """Swap between a dark and light theme when the app theme changes."""
+        self.theme = "vscode_dark" if self.app.dark else "github_light"
+
     def _on_mount(self, _: events.Mount) -> None:
         self.blink_timer = self.set_interval(
             0.5,
             self._toggle_cursor_blink_visible,
             pause=not (self.cursor_blink and self.has_focus),
         )
+        self.watch(self.app, "dark", self._retheme)
 
     def _on_blur(self, _: events.Blur) -> None:
         self._pause_blink(visible=True)