Grey bar or border

I am getting to know Customizr which I am trying out locally. There is a grey bar or border under the header/menu stretching across the whole page. I’ve been trying to find the CSS for it but I can’t find it in Inspect elements. For example I looked at:
#main-wrapper
but there is nothing about the border or divider or bar.

I would like to remove it. Could you please tell me which CSS is applicable to it?

Thank you in advance.