feat(focus-ring): added a new mixin so we can override just the focus-ring color

PiperOrigin-RevId: 443108205

Author: codr

packages/mdc-focus-ring/_focus-ring.scss

@@ -105,3 +105,18 @@ $container-outer-padding: 2px !default;
     }
   }
 }
+
+/// Customizes the color of the button focus ring.
+///
+/// @param $inner-ring-color [$inner-ring-color] - Inner focus ring color.
+/// @param $outer-ring-width [$outer-ring-width] - Outer focus ring width.
+@mixin focus-ring-color(
+  $inner-ring-color: $inner-ring-color,
+  $outer-ring-color: $outer-ring-color
+) {
+  border-color: $inner-ring-color;
+
+  &::after {
+    border-color: $outer-ring-color;
+  }
+}