The basics are simple, but if you want to do styling tweaks or anything approaching a "custom" UI, you need to start hacking away at an obscure C++ SDK or an even weirder mishmash of GDI+ and old JavaScript.
If there was a modern and easily customizable frontend to foobar2000's core audio and media library engine, I'd be in love. Something like Zeit's Hyper, but for music. I'd be willing to trade off Electron resource usage if it meant I could set up the UI just how I like it.