Well, I have to say that Harmonoid is the best one I've found, it has a ton of cool features, it works and it allows you to be able to see your collection by album and it doesn't use Electron. Right now its development has stalled a bit but it's still pretty much in good shape, though.
Which theme? If you configure it right it should match the system theme. Here's what mine looks like on Plasma 5, Breeze Dark.