Hi @memikee If you need this customization fleshed out then contact me by mail for services. ALWAYS please provide context, examples: store url, theme name, post url(s) , or any further detail in ALL correspondence. Contact info in signature.
The nav bar is surrounding by a header element and can't expand the full width of the page.
So you can color the nav elements background only without getting into more advanced CSS that's needs to be tinkered with.
You can see this by using a custom-css setting and the following Band-aid CSS: