Sign Up Buttons on WPMU Membership Suddenly Became Invisible

Hello there, on our site eatrightpittsburgh.org/register the buttons for WPMU Membership's "Sign Up" suddenly changed from green with a hover effect to white with a hover effect, effectively making them invisible. Im not sure if an update or something caused this, but if you could please advise some CSS changes to fix this I would be very grateful. Thank you!