add danmuku theme

docs/mobile.html

@@ -153,6 +153,7 @@
                     maxLength: 100,
                     minWidth: 150,
                     maxWidth: 0,
+                    theme: 'light',
                     beforeEmit: (danmu) => !!danmu.text.trim(),
                     // mount: document.querySelector('.artplayer-danmuku'),
                 }),