How to switch social icon colors

I'm over looking something here.

I am working with the Spirit theme. In the default theme the social icons located in the menu bar are solid white until hover then they turn dark.

I need to switch the social icons from static white to static dark then on hover switch to white.

I see there is a light and dark icon in the uf-spirit/images/sprite.png file, but when I try to switch css,. no luck

Can some one please advise the correct CSS.
Thanks
Mark