Centering menu

I recommend asking at https://wordpress.org/support/theme/primer so the theme’s developers and support community can help you with this. Also, posting a link to your site would be helpful.

remove all your other menu CSS with the ‘margin: auto;’

this might work:

.menu-primary-menu-container { 
display: block; 
text-align: center; 
}

#menu-primary-menu { 
display: inline-block; 
}