Google "open source music library software" and what is top of the list MusicBee which is definitely not open source!
As regards Music library + player,with MusicBee we are in a cathedral not a bazaar (see Eric Raymond). So if you want to do anything you have 2 choices:
Take hiccup's Sample Skin as a starter & change every one of the n elements to a colour of your own choosing or download one of @diego's skin and change what you want. Beware that, for example, there are are 64 (I joke not) named colour items for a skin that basically contains 3 separate colours - black gray & white.
My take: @hiccup was the pioneer. With his skin you could pinpoint what colour went with which UI element but it left unanswered the vital question - if panel x is coloured a,b,c what other panels are coloured a,b.c? In other words is there a object hierarchy or not? In a "clean" object hierarchy if the main window/panel is "x"then every other window/panel in the app would be in some way a descendent of "x". If you examine MusicBee with a UIA tool like UI Spy this is definitely not the case. As there is no documentation on the design, implemention, .., this is the only way to find out how it works. And if I discover that by doing z I can change MusicBee's behaviour how do I publish/share
As MusicBee is free at the point of use (like the NHS in the UK) we cannot complain about a free product. But if the source were available (with suitable checks) we could make MusicBee ours in the sense that we could tailor it to our own specific needs individually and generally.
As a starter would the moderator start a poll of how many year's IT experience members have/had?