We had this problem with our SharePoint theme where the text in the survey radio button’s would show up so light you couldn’t read them. Here’s how I fixed it.
In your theme, select the color setting for “Text/Backround – Dark 1” and change it to #000000 (black). Mine was set to #2F2B20 (which is like a really dark brown color). Looking at my original color you wouldn’t think that it would have such an effect on the text color but clearly it does. After the change voila! See before and after. This has been a problem for a while. Fixed now. 🙂