Add background and drop shadow to navigation toggle

Author: sunblaze

lib/rdoc/generator/template/darkfish/css/rdoc.css

@@ -241,6 +241,10 @@ nav a:hover {
   top: 10px;
   left: 20px;
   cursor: pointer;
+  background: var(--background-color);
+  padding: 0.3rem;
+  border-radius: 4px;
+  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.1);
 }
 
 #navigation-toggle[aria-expanded="true"] {