docs: add custom theme demo

Author: kagawagao

docs/demo/index.md

@@ -28,3 +28,12 @@ title: Demo
      allow="accelerometer; ambient-light-sensor; camera; encrypted-media; geolocation; gyroscope; hid; microphone; midi; payment; usb; vr; xr-spatial-tracking"
      sandbox="allow-forms allow-modals allow-popups allow-presentation allow-same-origin allow-scripts"
    ></iframe>
+
+## Use Custom Theme
+
+<iframe src="https://codesandbox.io/embed/g2plot-vue-use-custom-theme-0pggw?fontsize=14&hidenavigation=1&theme=dark"
+     style="width:100%; height:500px; border:0; border-radius: 4px; overflow:hidden;"
+     title="g2plot-vue-use-custom-theme"
+     allow="accelerometer; ambient-light-sensor; camera; encrypted-media; geolocation; gyroscope; hid; microphone; midi; payment; usb; vr; xr-spatial-tracking"
+     sandbox="allow-forms allow-modals allow-popups allow-presentation allow-same-origin allow-scripts"
+   ></iframe>