Added color themes (dark & light) (#148)

runtime/ui/index.html

@@ -16,9 +16,15 @@
             img {
                 display: block;
             }
+
+            @media (prefers-color-scheme: dark) {
+                html, body {
+                    background-color: #1a1a1a;
+                }
+            }
         </style>
     </head>
     <body>
-        <img src="icon.jpg" width="512px" height="512px" alt="The app logo">
+        <img src="icon.png" width="512" height="512" alt="The app logo">
     </body>
 </html>