Set z-index values

demos/assets/demo.css

@@ -13,6 +13,7 @@ body {
     height: 100%;
     object-fit: contain;
     opacity: 0.75;
+    z-index: 0;
 }
 
 #about, #sponsor {
@@ -21,6 +22,7 @@ body {
     height: 64px;
 
     opacity: 0.5;
+    z-index: 1;
     outline: none;
     -webkit-tap-highlight-color: transparent;
     cursor: pointer;