style.css, but honestly it would be way easier to just grab a new theme! But if you insist, a little googling always helps...
edit: ok so you asked "what part of the... style sheet" is responsible for text formatting and I said style.css so that's all pretty obvious, but if you don't know how to find what parts of the css to edit then you really should just use a different theme.
https://www.google.co.kr/search?q=ku...ient=firefox-a