fix(slider): use secondary custom property color for slider container (#5132)

* fix sass linting errors

* Fixed feature targeting test

* Rebase upstream

Author: svencowart

packages/mdc-slider/_mixins.scss

@@ -435,8 +435,16 @@
   $feat-color: feature-targeting-functions.create-target($query, color);
 
   .mdc-slider__track-container {
-    @include feature-targeting-mixins.targets($feat-color) {
-      @include theme-mixins.prop(background-color, rgba(theme-variables.prop-value($color), $opacity));
+	  @include feature-targeting-mixins.targets($feat-color) {
+      &::after {
+        @include theme-mixins.prop(background-color, $color);
+
+        display: block;
+        width: 100%;
+        height: 100%;
+        opacity: $opacity;
+        content: "";
+      }
     }
   }
 }