custom colour for the GUI

We have a large production system and for testing a small pre-production environment.
Both look the same except the small label in the upper-right corner, which could be customized.

Too often it happens, that we’re doing administrative things and then notice, we’ve been on the wrong platform.

The ability to change the look globally would be great, i.e. by changing the background colour of the top- and left-bar (status- and menue-bar)