Not possible to set background color of unchanged lines

It is not possible to change the background color of unchanged lines. I tried the following, which did not produce an error, but did not work either. ```(set-face-background 'git-gutter:unchanged "red")``` However all of these work: ``` (set-face-background 'git-gutter:modified "red") (set-face-background 'git-gutter:added "red") (set-face-background 'git-gutter:deleted "red") ```