I apologize if other people have already brought this up, but what is with the font colors? The headers of white type on a light blue background are bad enough, but at least you're not trying to study headers. But then I try to read the Tips and Notes -- my God! Light gray on a white background? Who came up with this? Is there a way to change it? If not in Duo, perhaps in Firefox?
If you’re familiar with CSS, you could examine Duolingo’s CSS to write an appropriate userstyle for changing the font colors; the userstyle would then work with any browser that supports the Stylus and Stylish browser add-ons. (Of course, you could first search the userstyles.org Web site to see if someone has already written such a userstyle.)