The look to a darker one
I would like to suggest changing the actual look of duolingo to a darker one, cause it would be more comfortable and healthy to learn looking at a little bit darker backgroud thank it is now.
It's just a proposition, I hope to draw a response from you, greetings.
Note that there's also a version where you can customize the color if you think the default color scheme is too dark: http://userstyles.org/styles/90750/duolingo-dark-customizable
PS: I am the author of the stylesheet :)