Edit colour of back to top arrow

Hi,

First of all, great theme guys! Love it!

The only issue I’m having is the “back to top” arrow that appears in the bottom right of the screen. It’s currently a white arrow on a yellow background, and I want to change it to a white arrow on a green background.

I can’t see any option for this so I’m assuming it’s a custom CSS job – how would I change it here?

Thanks!