Change responsive font size?

Dear friends,

I am currently using Mantra Theme without being good at html or css, but I nearly solved most of the problems with the help of you guys out there.
Is it possible to change the font size on mobile devices? I activated responsiveness, header and pictures are fine, but the text in the articles is far too big, at least on safari iPhone. Is there a simple way editing the mobile.css or so?

Thank you very much in advance
Sandman