Emphasize "Mark as Complete" button for excercise on webpage

Hi, I noticed it’s already mentioned in github issue #5928, but no here.

Just today I was looking why my exercise is not marked as completed and after some time I discovered “Mark as complete” button. It took me a while to locate it. The button is white on white background. Not really visually distinctive IMHO.

I’m hoping this can be resolved sooner, than later as it’s a simple choice of better CSS colour for the button to visually emphasize it from the rest of the page.

Also it’s not clear if mentoring is mandatory or optional to finish the exercise to move on next one (I now know it’s optional)

Thank you.