[Resolved] no background colour on mobile header

Hi,
I have created a mobile menu and deactivated the off screen panel.
The navigation and header background colour works fine on desktop:

On mobile the header and navigation have a white background:

I tried deactivating GeneratePress premium plugin. The header reverted to the desktop header with the site title and byline. It seems that there is a problem with the background colour of the mobile menu:

Is there a way of changing the header/navigation background colour in mobile view so that it matches desktop view, while displaying the mobile menu correctly (without the byline)?
If no, should is .mobile-header-navigation the correct css selector to use, to change the header/navigation background?