Added header_color to change the color of the header

This PR add a new key to the app.yml: header_color in order to tune a bit the display.

The idea is to create a kind of identity per beamline or session, which could also help to avoid mistakes.

Here is the result.

Another possibility could be to let custom a default.css file in the resources directory. This would be more flexible.

I am fine with both solution.