Hello, Dear Ben!

Thank you for your question.

To make header full width, please click WP Dashboard => Appearance => Customize => Additional CSS and paste this code here:

@media (max-width: 1920px) {
.page_header [class*="container"] {
    transition: padding-top 0.2s linear 0s, padding-bottom 0.2s linear 0s;
    width: 1800px;
    }
}

Then save changes and check results at front-end:

https://prnt.sc/Youjb9lYhiyM

Best Regards!