fix color styles are broken when system is in light mode

rhysd · rhysd · commit ccaabc88e60b · 2024-06-09T20:03:44.000+09:00
diff --git a/playground/index.html b/playground/index.html
@@ -1,5 +1,5 @@
 <!DOCTYPE html>
-<html lang="en">
+<html lang="en" data-theme="dark">
   <head>
     <title>actionlint playground</title>
     <meta charset="utf-8"/>
