Applying code to style.css

Hi there, I'm using a WP theme for a site, but it won't allow users to re-size fonts. This is a problem from an accessibility standpoint.

I noticed you said, "instantiate the class on any page where you want to use the resizer" which makes me think it has to be done on a page by page basis. Can I can apply this to one my theme's style sheets so it takes effect across the entire site?

Sorry if this is a newbie question. I'm not great with coding, but understand enough to get a job done :)

Thanks

Sandi