DEPRECATED - Firefox functionality will eventually be rolled into Web Extensions version. Find the new repo here: https://github.com/chrisxclash/play-midnight
Play Midnight is a different take on the standard theme that is used on Google Play Music. As much as I love the original look of Play, the brightness can hurt the eyes after a while. After noticing there wasn't a dark alternative Play Midnight came to be.
The Firefox Addon for Play Midnight uses CSS generated via SASS for the different color options. These files are all self-contained in the /sass folder and should be commented somewhat to help you understand where each of the settings are.