I am trying to change the font size on all pages/posts on the site I’m managing for my employer. I know how to change the font size in “posts” but how do I change font size in “category” pages?

I have tried looking into appearance editor to change the font size.

I looked online at different codes to insert but it is unclear where to place the code or what follow on steps must be done.

This is one of the pages I am trying to change: