I need help with custom css for mobile squarespace

Hi

I've just gotten into brand design in the last year and Im building a website for a client on squarespace at the moment but Im coming up against issues when the site is viewed on mobile device. As id love to learn custom css for squarespace I suppose Im looking for someone who can help me adjust the code on the site so it looks good on mobile but doesn't effect the desktop view, and I'd like if they were open to explaining to me the code to do this (rather than just doing it and Im none the wiser as to how it was done) if that makes sense.

Thanks
Naomi