Help creating full-screen mobile nav that not cover the header


Has anyone successfully edited the mobile nav while it’s open to fill up the entire screen and not covering the header?

This is the link for my website.

Looking at the picture below, this is the current status. I found a way to make it full screen, but then it covers the whole header in white, but I don’t want it to be covered by while a user click on the menu button on mobile mode.

Ideally, it would fill up the entire screen and, without covering the header like this:

Any help would be much appreciated!