Background of frames.
What I think would be great is customisable backgrounds. Personally I like a kind of natural look .. like a parchment with sepia tones but everyone will be different so how about a choice of backgrounds.
Could be applied to the background of both the inner frame (if frames are what you are using) and the main background. This could be easier on the eyes too as the white background on the inner frame is ok for a while but if you want to do a long session on duolingo then anything with less glare that makes it easier on the eyes is good.
I used to play an online game called Utopia and the alliance I belonged to had a website with a black background and you could stare at it all day with no eye strain at all.