fix(slider): thumb label blending in with background in high contrast mode (#12606)

Fixes the slider's thumb label not having an outline and blending in with the background in high contrast mode.

Author: crisbeto

src/lib/slider/slider.scss

@@ -142,6 +142,10 @@ $mat-slider-focus-ring-size: 30px !default;
   transition: transform $swift-ease-out-duration $swift-ease-out-timing-function,
               border-radius  $swift-ease-out-duration $swift-ease-out-timing-function,
               background-color  $swift-ease-out-duration $swift-ease-out-timing-function;
+
+  @include cdk-high-contrast {
+    outline: solid 1px;
+  }
 }
 
 .mat-slider-thumb-label-text {
@@ -339,6 +343,13 @@ $mat-slider-focus-ring-size: 30px !default;
     .mat-slider-thumb-label {
       transform: rotate(45deg);
     }
+
+    @include cdk-high-contrast {
+      .mat-slider-thumb-label,
+      .mat-slider-thumb-label-text {
+        transform: none;
+      }
+    }
   }
 }