Don't overwrite background-image etc when setting background-color

[M393]

I use buttons with image background, but the background gets overwritten by the css in Bar.css because it uses the background style instead of background-color This unfortunately also resets background-image / background-position/ background-repeat and a few more styles to their defaults.

It should be enough to only set the background-color here.

This PR deals with toggle buttons only but there's a lot of other places where background is used instead of background-color, maybe consider changing those too.