How to change MarketPress checkout button colour?

I've been trying to change the checkout button from green to another colour... searched the forums (e.g. https://premium.wpmudev.org/forums/topic/how-do-i-change-the-color-of-the-check-out-now-image-in-marketpress), downloaded the Simple Custom CSS plugin, but it just doesn't work.

Is there a quick fix to this?