CHANGING FONT STYLE

I’m using a Weaver II theme and the default font style appears to be Times New Roman. I want to change the font style on my various pages to Helvetica or some other sans serif font. I’ve researched how to do this on the internet and I understand there is a CSS code change needed – I can pass a tutorial test. Unfortunately, I can’t figure out where in my WordPress website program I need to make this code change. In the page edit mode I see code in the “Text” tab, but when I insert text style code there it shows up verbatim on my page – no change to font style. I tried to customize Weaver II, but I can’t see where the font style section is. I’m stumped.