Always open on mobile only

I think it's a bad, and strange, request, but I have one client wanting the menu to be always expanded on mobile. I'm sure it's really obvious, but I've looked and looked and cannot see where to set this.

Hi
what do you mean with "open" ? do you want the whole menu to be shown by default when the page loads ?
you shall be able to do that with CSS
CEd

Yes, he wants the entire menu to show - I hate the idea and tried to talk him out of it. However, he didn't know what the hamburger menu was so immediately assumes everyone else is just as illiterate!!

And Happy New Year Ced!

thank you, happy new year to you too !

Can you give me a hand with the CSS I need please?

give me the url of the page