feat(ui): implement CSS variables for theme support (#26)

#17

Reviewed-on: #26
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/light-mode.less

@@ -1,2 +1,4 @@
-@background-image: url('/assets/images/img.png');
-@color: black;
+:root {
+  --background-image: url('/assets/images/img.png');
+  --color: black;
+}