Restructure Sass files for faster loading.

src/style.scss

@@ -1,4 +1,7 @@
-* {
+body {
+  margin: 0;
+  overflow: hidden;
+  text-align: center;
   font-family: verdana, sans-serif;
 }
 
@@ -9,5 +12,52 @@
   left: 0;
 }
 
-@import "open";
-@import "closed";
+// Displays something full-screen
+@mixin full-screen {
+  position: absolute;
+  top: 0;
+  left: 0;
+  width: 100%;
+  height: 100%;
+}
+
+// Styles for the "Open" page
+canvas {
+  @include full-screen;
+  background: #000;
+}
+
+// Styles for the "closed" frame.
+@keyframes fadeOut {
+  0% {
+    opacity: 1;
+  }
+
+  100% {
+    opacity: 0;
+  }
+}
+
+body.closed {
+  background: #f66;
+
+  .closed_notice {
+    position: absolute;
+    font-weight: bold;
+    font-size: 16pt;
+    text-transform: uppercase;
+    animation: fadeOut 3s forwards;
+    animation-timing-function: linear;
+    animation-iteration-count: 1;
+  }
+
+  #stop-overlay {
+    @include full-screen;
+    z-index: 1;
+    background-size: contain;
+    background-repeat: no-repeat;
+    // File is served from a subdirectory.
+    background-image: url(../stop.svg);
+    background-position: 50% 50%;
+  }
+}