When I went looking for a dark theme for Google Music, all of them was out of date and broken, so with no other option I made my own. I personally use this theme so you can always count on it being updated when it needs it, and when I get bored maybe clean things up a little.

Problem? Screenshot it and post it in the issues thread below or email it to me with the browser you are using.

Google Play / Music Dark theme by Rsslone.
Email : Rsslone1120@gmail.com

Changes:

4/18/16- Added webkit code for scroll bar skin.
4/16/16- Fixed a problem with z-index not applying properly.
4/8/16- Fixed scroll bar being overlapped by artist/album image container.
4/8/16- Fixed a big problem where the background from sidebar was bleeding into the main container on artist/album pages. Also add/fixed the artist images. (known problem of the image overlapping the scroll bar, working on it)
4/6/16- Fixed where the background image would bleed into the sidebar
2/8/16- Updated Search Bar
2/7/16- Updated scroll bar to match, updated screenshots
2/3/16- Changed Equalizer to white instead of black. Lightened up some text for readability.
10/27/15- Fixed broken background, header, and sidebar.
7/12/15- Fixed album view boxes / numbers showing though play button
7/2/15- Brightened up text to fix hard to read titles. Thanks Jesse Sandoval for the suggestion.
6/23/15- Fixed some panel colors from google update
6/5/15- Fixed playlist row color
5/27/15- Fixed hard to read text on radio page
5/23/15- Fixed radio elements (Thank you Frazer for the help)
5/16/15- Theme entirely redone due to google music UI overhaul. (Report bugs/issues please)

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5/4/15- Fixed issue with images and colors behind logo + Fixed search bar (thanks GearHead for the catch)
3/18/15- Fixed player button transparency
3/7/15- Fixed header transparency + header buttons spacing
3/3/15- Fixed header transparency + fixed broken thumbnails
2/20/15- Fixed Playlist box to match theme
2/19/15- Fixed transparency on load issue / darkened icons / Changed loading background
2/7/15- Update required for changes made by google / Modified upload box to match
2/2/15- Changed "uploading music" notification to match
1/28/15- Fixed player bar to match theme, fixed possible issues when loading theme.
1/27/15- Changed header/player colors to match scheme
1/14/15- Fixed header/title transparency
12/9/14- fixed music tile transparency
12/5/14- fixed header transparency