Dear,
I wanted to change the fonts of the site, but I don't want to change one by one.
How could I change all of them?

Or to change the fonts of the large contents...

Hi
You can change the fonts (both style and size) in the template css. Some templates allow you to do this in the template parameters so it is worth checking there first.
Otherwise firebug is a good tool to find out where the fonts are located within the css files.
www.ostraining.com/blog/joomla/using-fir...it-joomla-templates/
Each set of fonts is listed separately ie menu, headings (h1, h2 etc), paragraphs, links etc.
Give firebug a try we can give more help if your site is live.
Regards