Enable dark mode in browser UI (#4089)

beta/src/styles/index.css

@@ -17,10 +17,16 @@
 
   /* Write your own custom base styles here */
   html {
+    color-scheme: light;
+    
     -webkit-tap-highlight-color: rgba(0, 0, 0, 0);
     -webkit-tap-highlight-color: transparent;
   }
 
+  html.dark {
+    color-scheme: dark;
+  }
+
   html,
   body {
     height: 100%;

beta/src/styles/sandpack.css

@@ -3,6 +3,8 @@
  */
 
 html .sp-wrapper {
+  color-scheme: inherit;
+  
   /* palette */
   --sp-colors-fg-active: #24292e !important;
   --sp-colors-fg-default: #959da5 !important;