Can I change the color of the Mark as Completed button?

To change the color of the Mark as Completed button:

First you can use the Customizer:

1. Click on WP Courseware→Settings.
2. Click on the Style tab.
3. Click the Yes radio button for Customize Colors.
4. Click the Unit Colors menu item on the left.
5. Modify any of the colors for the Unit Complete Box.

You could also click the Use Customizer button and edit the same properties via the frontend customizer.