Harmony is based on plugins. Plugins are available for Spotify, SoundCloud, Google Play Music, Hype Machine, Deezer, and local files.
Quickly change to darkmode by pressing Ctrl/Cmd+D
Whatever size you want it to be, it will follow you
Use the system tray or the sound menu integration (Linux) to control the playback even when the app isn't focused.
Harmony natively supports Last.fm scrobbling