Further to Bee-liever's comment it is relatively simple to add another "modern" touch to MusicBee. I read an article (sorry can't remember where) which said that app developers start with a mobile phone app and if it works on a small device they can easily scale up. One feature of mobile apps is that it's "tap & go" our equivalent is click & go). An easy implementation on MusicBee for, say, Windows buttons Minimize, Maximize/Resize and Close is to make the highlight image the same as the "normal" icon (change MaximizeHightlight.png to Maximize.png). The same works for icons on Flat player.
However this might not appeal to the "traditionalists" so I would suggest a setting in <settings> something like:
<element id ="WindowsModernStyle">true/false </element>
the effect of which if true is to load only the base images (are there disabled images?) and ignore those with "Highlight" embedded