Unfortunately, it's impossible to change fonts in Safari 6. The option of changing fonts was available in the previous version through Safari (menu bar) > Preferences > Appearance. It seems that Apple removed this option and now it's impossible to select custom fonts for the application or websites you visit.
I recommend posting a polite request using Safari Feedback. This way Apple will know there are users who want to change fonts in Safari.
Feedback: http://www.apple.com/feedback/safari.html
