Move Toggled Menu Below Header

Great starter theme, I appreciate your hard work. I am testing my site locally, so I cannot share a link for viewing.

I fixed the masthead to the top and gave it a background image (150px height) and the navigation resides inside of the masthead’s container. When toggled, the menu pushes the masthead down, along with the masthead’s background image. I would like the toggled button to reach 100% width and no push the image down, OR move the menu below the masthead altogether. No matter what I tried, I could not get either way to work properly. I’m assuming it’s something beyond the stylesheet. Any help would be appreciated!