fix(tab): wrong button color in fluent theme (#12390)

Co-authored-by: Simeon Simeonoff <[email protected]>

Author: SisIvanova

projects/igniteui-angular/src/lib/core/styles/themes/schemas/light/_tabs.scss

@@ -159,7 +159,7 @@ $fluent-tabs: extend(
         ),
 
         button-disabled-color: (
-            color: ('gray', 500)
+            color: ('gray', 400)
         ),
     )
 );