add danmuku theme

packages/artplayer-document/docs/en/Plugins/danmuku.mdx

@@ -89,6 +89,7 @@ var art = new Artplayer({
             maxLength: 100, // The maximum number of words that can be input can be input box
             minWidth: 200, // At the at least width of the input box
             maxWidth: 400, // Maximum width in the input box
+            theme: 'dark', // The theme color of the input box
             beforeEmit: (danmu) => !!danmu.text.trim(), // Custom verification before the barrage emit
             mount: undefined, // Customized input box mount position
         }),