center, full sized logo

There are many snippets to this theme;

https://www.themesandco.com/snippet/center-logo/

I must be doing something wrong, I see no changes after adding the CSS.

That snippet is php, not CSS. The code needs to be added to a Child Theme functions.php

See Guide

Aaand that solved that, thanks! Would it too much to ask how to center the navbar underneath it?

That’s all covered in a different snippet, slightly different to the previous one.