Hi there,
the Read Site only centers the header on screensizes between 1040px and 1111px and it uses the following settings in Customizer > Layout > Primary Navigation.
1. Navigation Location: Float Right
2. Navigation Drop Point set to 1111px – this is the width of the screen where the Navigation drops below the logo, and the site header is centered. you would need to tweak that value to suit your site.
Alternatively if you want to center your branding and navigation you can do this:
1. Customizer > Layout > Header –> Alignment: Center.
2. Custimizer > Layout > Primary Navigation –>
2.1 Navigation Location: Below Header
2.2 Navigation Alignment: Center