Make Header Full-Width

[hcotto]

Hello! I have been looking through the forums for the right way to do this, but nothing seems to be working.

I have very little experience with CSS, and am only learning now, so I am not sure if maybe what I wan requires more than just simple CSS.

This is the website I am working on - https://stevemcgill.com/

 

I currently have this 'additional CSS' installed on the above page, but I can not get the header to go full-width.

a{
    background-color: #363636;
     border-bottom-style: solid;
}
li.hurdlers-hurdler a{
border-bottom-color: coral}

li.art-hurdling a{
    border-bottom-color: aqua}

li.wip a{
    border-bottom-color: blue}

li.poet a{
    border-bottom-color: orange}

li.essays a{
    border-bottom-color: green}

li.stories a{
    border-bottom-color: yellow}

li.dreams a{
    border-bottom-color: violet}

li.meditation a{
    border-bottom-color: pink}

li.shorter-w a{
    border-bottom-color: red}
 

I am trying to get the look of the header on https://nicholassparks.com/

 

Any help would be greatly appreciated!

 

Thank you!