Fix static CSS tests

Author: Nazar65

app/code/Magento/MediaGalleryUi/view/adminhtml/web/css/source/_module.less

@@ -18,7 +18,7 @@
 @color-media-gallery-buttons-border: #adadad;
 @color-media-gallery-buttons-text: #514943;
 @color-media-gallery-checkbox-background: #eee;
-@color-media-gallery-scrollbar-background: white;
+@color-media-gallery-scrollbar-background: #FFFFFF;
 & when (@media-common = true) {
 
     .media-gallery-delete-image-action,