Use the theme installer to install this.
Alternatively, you can download the latest release from here and replace the files in your Apps folder.
On macOS the Apps folder is inside the app: /Applications/Spotify.app/Contents/Resources/Apps.
On Linux the Apps folder is located in /usr/share/spotify/Apps.
On Windows the Apps folder is located in C:\Users<username>\AppData\Roaming\Spotify\Apps.