Header nav colours appear to be broken

Hi, thanks for making such a great theme :) Just wanted to give you a heads up that the nav "disappears" after you click on any option, as they're staying white on a white background. I thought I'd maybe broken it, but it looks that way on the Grav Themes page too.

https://demo.getgrav.org/agency-skeleton/#portfolio

Thanks again, let me know if it's an easy fix!

Guy