Sign Up Button Styling

Sorry to ask this (as I’m sure it’s incredibly simple) but I’ve tried every combination I can think of, with the use of Firebug, to change the colour of the Sign Up button located in the top right hand corner of the Whoop header but with no luck. I’ve been trying to add Custom CSS as the button styling options within Whoop Options don’t seem to apply to this particular button but nothing I add makes any difference to this button. Any help with this would be greatly appreciated.

Thanks,

T