add doc for dark mode

docs/source/dark.md

@@ -0,0 +1,17 @@
+# Dark Mode
+
+This package has theme switching functionality. You can switch between light and dark mode by clicking the button in the top right corner of the page.
+
+It will set something like `data-theme="dark"` to the root HTML element.
+
+## Tailwind CSS
+
+If your project use Tailwind CSS, please config `tailwind.config.js`
+
+```js
+module.exports = {
+    darkMode: ["selector", '[data-theme="dark"]'],
+}
+```
+
+If TailwindCSS detect `data-theme="dark"` in the root HTML element, it will apply dark mode styles.

docs/source/index.rst

@@ -13,4 +13,5 @@ Topics
    install_with_viewcomponent.md
    write_preview.md
    params.md
+   dark.md
    contributing.md