Style: Dark mode body on load alternative (#8287)

## Summary

Alternative to https://github.com/Comfy-Org/ComfyUI_frontend/pull/8077

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8287-Style-Dark-mode-body-on-load-alternative-2f26d73d365081fb8231f167e75beb83)
by [Unito](https://www.unito.io)

index.html

@@ -10,7 +10,13 @@
     <meta name="mobile-web-app-capable" content="yes">
     <!-- Status bar style (eg. black or transparent) -->
     <meta name="apple-mobile-web-app-status-bar-style" content="black">
-    
+    <style>
+      @media (prefers-color-scheme: dark) {
+        body {
+          background-color: #202020;
+        }
+      }
+    </style>    
     <link rel="manifest" href="manifest.json">
   </head>
   

manifest.json

@@ -11,6 +11,5 @@
     }
   ],
   "display": "standalone",
-  "background_color": "#ffffff",
   "theme_color": "#000000"
 }

packages/design-system/src/css/style.css

@@ -584,8 +584,6 @@ body {
   height: 100vh;
   margin: 0;
   overflow: hidden;
-  background: var(--bg-color) var(--bg-img);
-  color: var(--fg-color);
   min-height: -webkit-fill-available;
   max-height: -webkit-fill-available;
   min-width: -webkit-fill-available;