Thanks for the link. Adding the below as is to your piggy back file should fix it:
Arsenic.xmlc is a relatively old skin by Bee-liever if I'm not mistaken.
There are some elements in MusicBee that are dependent on each other to a point where if one is missing, a whole panel may start behaving funny.
In this case, there are a couple of elements listed below that are missing in the original skin file. Simply adding the .Lowlight by itself won't affect anything.
<element id="NowPlayingList[TrackDetail].ListHeader" bg="59,68,75" fg="138,148,156" bdr="103,118,129" />
<element id="NowPlayingList[TrackDetail].Disabled" bg="59,68,75" fg="138,148,156" bdr="59,68,75" />
<element id="NowPlayingList[TrackDetail].ListAlternating.Disabled" bg="59,68,75" fg="138,148,156" bdr="59,68,75" />
<element id="NowPlayingList[TrackDetail].Default" bg="59,68,75" fg="138,148,156" bdr="59,68,75" />
<element id="NowPlayingList[TrackDetail].ListAlternating.Default" bg="59,68,75" fg="138,148,156" bdr="59,68,75" />
<element id="NowPlayingList[TrackDetail].Lowlight" bg="59,68,75" fg="138,148,156" bdr="59,68,75" />
<element id="NowPlayingList[TrackDetail].Highlight" bg="36,41,46" fg="166,171,176" bdr="163,171,176" />