Revert #6733 to avoid breaking changes for extensions (#6821)

This reverts commit 4af8fbaa34.

Signed-off-by: Alex Andreev <alex.andreev.email@gmail.com>

Signed-off-by: Alex Andreev <alex.andreev.email@gmail.com>

src/renderer/components/select/select.scss

@@ -145,7 +145,7 @@ html {
   //-- Themes
 
   .Select__menu {
-    z-index: var(--z-index-menus); // render at the top when used inside dialog
+    z-index: $zIndex-select-portal; // render at the top when used inside dialog
   }
 
   .Select, .Select__menu {