fixed accessibility issues when assigning courses to a colour group, fixes #4047

Closes #4047

Merge request studip/studip!2902


(cherry picked from commit e1c2f5b8)

31f436ae fixed accessibility issues when assigning courses to a colour group
dbc159c8 changed coloured box design
9bdf38d3 Apply 3 suggestion(s) to 2 file(s)