Change Menu Font

Various ways, but I think this would work:

#menu_wrapper {
    font-family: ...
}

Ideally you would add it to the end of a child theme style.css file that imports all the styles from the parent theme.

But if you want to modify the theme in situ, you will need to add the rule above either to the end of this style sheet:

https://www.onebuildingservices.co.uk/wp-content/themes/Hermes/css/screen.css?ver=1.0

… or to any style sheet that gets loaded after that one.

HTH

PAE