Finally got it.
For anyone with the same problem, here’s what I did:
in the _768.less file (which affects the style of the site when dealing with desktop widths),
— under the .nav class:
width: 440px; /* the width of the whole menu */
margin: 0 auto;
— under the li element:
width: 110px; /* width of each item */
It is not the solution I wanted — I’d like an automatic centering solution without specifying the width of the menu — but it simulates it somewhat.
Of course if I add a new menu item, I’ll have to tweak the widths, unfortunately…
