You’ve probably noticed that modern websites often use simple, frame-only buttons over full-screen graphics (or even videos). We use it on our website:
It’s easy to use the same design in your course, too! Simply set the fill to transparent, set the button border to white 3px, choose a modern font (Open Sans is a great free modern font) and you’re ready to go.
For the hover state, simply invert the colors by setting the fill to solid white and text to black.
Hope this helps!
P.S. Here’s a bit more inspiration for flat modern design: http://community.trivantis.com/knowledge-base/minimalist-design/