More stepper styling

[miggol]

https://utrechtuniversity.github.io/UU-Bootstrap/components/stepper/

In the reference and procreg steppers, there are green and yellow stepper bubble classes that have not yet been implemented here in Ethics.

This should not be hard, and can be implemented once in the base stepper item class. Though #693 should probably be completed first so the errors implementation is complete.