Papillon – Menu underline

I am using the Papillon theme. Seems like a good theme but whatever I do I can’t seem to change the colour of the red underline on the default header menu.

I’ve found the colour is #f63b29 and have searched the CSS sheet for all instances of it but changing any of these has no effect.

Before that I went through all the theme options and did find things which seemed to refer to it but changing these didn’t have any effect either.