fix(ui): added dark mode (#25)

Reviewed-on: #25
Reviewed-by: lq64 <lq@blackhole.local>
Co-authored-by: Janis <janis.e.20@gmx.de>
Co-committed-by: Janis <janis.e.20@gmx.de>

knockoutwhistweb/app/assets/stylesheets/dark-mode.less

@@ -0,0 +1,2 @@
+@background-image: url('/assets/images/background.png');
+@color: white;

knockoutwhistweb/app/assets/stylesheets/main.less

@@ -98,7 +98,7 @@ html, body {
     height: 100%;
 }
 #rules {
-    color: white;
+    color: @color;
     font-size: 1.5em;
     font-family: Arial, serif;
 }