Allow different background colour in builder

I copied an app to create a 2.0 version. I need to work every day in both apps because our staff are using 1.x and I am actively developing and testing 2.0.

The issue is the two apps look very similar. It is far too easy to make a mistake and think I'm editing 2.0 when I'm actually in 1.x

I want to be able to change the app background colour in the builder so that it is much clearer which app I am working in.

For clarity - I mean changing the colour of the header background of the builder interface itself. Custom colours instead of Knack purple.