Update splash screen

web/index.html

@@ -34,6 +34,11 @@
   <link rel="stylesheet" type="text/css" href="splash/style.css">
 </head>
 <body>
+  <picture id="splash">
+    <source srcset="splash/img/light-1x.png 1x, splash/img/light-2x.png 2x, splash/img/light-3x.png 3x, splash/img/light-4x.png 4x" media="(prefers-color-scheme: light) or (prefers-color-scheme: no-preference)">
+    <source srcset="splash/img/dark-1x.png 1x, splash/img/dark-2x.png 2x, splash/img/dark-3x.png 3x, splash/img/dark-4x.png 4x" media="(prefers-color-scheme: dark)">
+    <img class="center" aria-hidden="true" src="splash/img/light-1x.png" />
+  </picture>
   <!-- This script installs service_worker.js to provide PWA functionality to
        application. For more information, see:
        https://developers.google.com/web/fundamentals/primers/service-workers -->

web/splash/style.css

@@ -2,7 +2,7 @@ body, html {
   margin:0;
   height:100%;
   background: #000000;
-  
+  background-image: url("img/light-background.png");
   background-size: 100% 100%;
 }
 
@@ -37,7 +37,7 @@ body, html {
     margin:0;
     height:100%;
     background: #000000;
-    
+    background-image: url("img/dark-background.png");
     background-size: 100% 100%;
   }
 }