Created a mixin for ACX's material spinner that allows customization of the spinner's thickness.

PiperOrigin-RevId: 207115306

Author: Googler

lib/material_spinner/_mixins.scss

@@ -0,0 +1,10 @@
+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Changes the border-width of the spinner so it appears thicker or thinner.
+@mixin material-spinner-thickness($stroke-width) {
+  ::ng-deep .circle::before {
+    border-width: $stroke-width;
+  }
+}