Make track's hightlighted crates more obvious

From https://bugs.launchpad.net/mixxx/+bug/1380467 :

At the moment (with the Latenight skin at least), crate names that a track is in are just bolded, and that's not very easy to notice, unless you're really looking for it. It would be much better if the crate's background was highlighted, perhaps in a similar way to when the mouse hovers over it (maybe a different colour though). I don't know if this is a problem with the skin itself, or the skin engine in general. There may be other ways to make it more obvious - bold-italic, underline, or an icon next to the crate name.