Forcefully set trackchanges textareas background to white.

services/web/public/stylesheets/app/editor/review-panel.less

@@ -571,6 +571,7 @@
 		border: solid 1px @rp-border-grey;
 		resize: vertical;
 		color: @rp-type-darkgrey;
+		background-color: #FFF;
 		margin-top: 3px;
 		overflow-x: hidden;
 		min-height: 3em;