"Call-to-action" buttons on beta sites such as the "join site" buttons on the homepage and the small banner on top of questions pages when you're not logged in are currently styled with an orange background and white text. This changes to a darker orange background with a complementary dark blue text color when hovered.
This is not ideal or the easiest combination to see. I recommend the white text color be kept in the hover style.
Giant images for your demonstration pleasure (I'm not at home as I post this, so I haven't had chance to freehand circle or crop these, which I will do later on):