[Feature Request]: Background Colour Configuration

### What would you like to happen?

Would it be possible to change the colour of the Hop UI background using a colour picker?

Would be even better to define a colour per Project / Environment. (Red for production, blue for development etc)

### Issue Priority

Priority: 3

### Issue Component

Component: Hop Gui

Start in the Hop GUI and trace how background appearance and Project or Environment configuration are currently represented. Clarify whether the request requires a global colour picker, per-Project or per-Environment values, or both, then define the expected behaviour for switching contexts and confirm it in the GUI.

Written by the indexing model from the issue text.