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/dock/dock.scss

@@ -20,10 +20,8 @@
  */
 
 .Dock {
-  $borderColor: $borderColor;
-
   position: relative;
-  background: $dockHeadBackground;
+  background: var(--dockHeadBackground);
   display: flex;
   flex-direction: column;
 
@@ -62,7 +60,7 @@
 
   .tabs-container {
     padding: 0 $padding * 2;
-    border-top: 1px solid $borderColor;
+    border-top: 1px solid var(--borderColor);
     flex-shrink: 0;
 
     .Tabs:empty + .toolbar {
@@ -78,7 +76,7 @@
 
   .tab-content {
     position: relative;
-    background: $terminalBackground;
+    background: var(--terminalBackground);
     flex: 1;
     overflow: hidden;
     transition: flex-basis 25ms ease-in;