Replacing global themed sass-vars to css-vars (#4322)

* generating css-vars
* convert all global / theme sass vars to css vars with available default theme colors
* added missing colors to dark lens theme

src/renderer/components/cluster-manager/cluster-manager.scss

@@ -53,7 +53,7 @@
     right: 0;
     bottom: 0;
     display: flex;
-    background-color: $mainBackground;
+    background-color: var(--mainBackground);
 
     iframe {
       flex: 1;