[Toolkit] Update logopage styling.

Part of the UX refresh Pale Moon effort. This uses the new -moz-dark-theme media
query to make the logopage dark if the UI is dark, without explicitly relying on
(often not sane) system color values.

toolkit/content/logopage.xhtml

@@ -13,8 +13,8 @@
   <title></title>
   <style type="text/css">
   body {
-    background: Menu;
-    color: MenuText;
+    background: rgb(229,229,229);
+    color: black;
   }
   
   img {
@@ -27,6 +27,13 @@
     left: 0;
     opacity: 0.2;
   }
+  
+  @media(-moz-dark-theme) {
+    body {
+      background: #2E3B41;
+      color: #eee;
+    }
+  }
   </style>
 </head>