Revert "Do not apply the receiver's color as background color (#864)"
Revert "Do not apply the receiver's color as background color (#864)"

This reverts commit 8ad24d996bda57290fc61fe808b3fa9b6f3569ba.

Unfortunately it has unwanted side effects (all avatars lose background
color).

threema-ch/threema-web,threema-ch/threema-web,threema-ch/threema-web,threema-ch/threema-web,threema-ch/threema-web