Same issue as CMurphy, tried the code listed below but then the navigation was no longer centred.
//
.header-nav {
width: 80% !important;
flex: 1 1 80% !important;
}
To me it's definitely the issue with the logo + button flex boxes being set to 33% each which seems unnecessarily large to me. Like why is there only 34% width set to nav items?!?!?!