Proposal to add custom themes to the editor

## Problem this feature should fix
Not 100% sure if this exists or not, but if would be nice if there was a way to either download or manually set custom editor themes. Ideally if it could also be reused in games later.

## Expected solution
In the settings somewhere to have a setting to set theme file, or a folder with themes where you can add your own theme (ideally for project manager also)
