Merge pull request #484 from ckeditor/t/ckeditor5/1662

Other: The `_rwd.css` mixin was moved to this package from `@ckeditor/ckeditor5-theme-lark`. See ckeditor/ckeditor5#1662.

Author: Reinmar

theme/mixins/_rwd.css

@@ -0,0 +1,10 @@
+/*
+ * Copyright (c) 2003-2019, CKSource - Frederico Knabben. All rights reserved.
+ * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-oss-license
+ */
+
+@define-mixin ck-media-phone {
+	@media screen and (max-width: 600px) {
+		@mixin-content;
+	}
+}