fix(tabs): change color for disabled tab label (#3483)

Fixes #3481

src/lib/tabs/_tabs-common.scss

@@ -14,7 +14,6 @@ $mat-tab-animation-duration: 500ms !default;
   font-weight: 500;
   cursor: pointer;
   box-sizing: border-box;
-  color: currentColor;
   opacity: 0.6;
   min-width: 160px;
   text-align: center;

src/lib/tabs/_tabs-theme.scss

@@ -26,4 +26,12 @@
   .mat-ink-bar {
     background-color: mat-color($primary, 500);
   }
+
+  .mat-tab-label, .mat-tab-link {
+    color: currentColor;
+
+    &.mat-tab-disabled {
+      color: mat-color($foreground, disabled-text);
+    }
+  }
 }