Doing what Chudo has suggested to get a mobile view, it looks like you're looking for this chunk of css? It would seem that new (unread) message backgrounds have been set to transparent. When i turn off that property in firebug, it defaults to a grey background. Are you looking to go back to that behavior?
#lia-body .lia-content .lia-component-message-list .lia-list-wide .lia-list-row-thread-unread td, #lia-body .lia-content .lia-component-message-list .lia-list-slim .lia-list-row-thread-unread td, #lia-body .lia-content .lia-component-notes-list .lia-list-wide .lia-list-row-thread-unread td, #lia-body .lia-content .lia-component-notes-list .lia-list-slim .lia-list-row-thread-unread td, #lia-body .lia-content .lia-component-kudos-list .lia-list-wide .lia-list-row-thread-unread td, #lia-body .lia-content .lia-component-kudos-list .lia-list-slim .lia-list-row-thread-unread td, #lia-body .lia-content .lia-component-bookmarks .lia-list-wide .lia-list-row-thread-unread td, #lia-body .lia-content .lia-component-bookmarks .lia-list-slim .lia-list-row-thread-unread td {
background-color: transparent;
}