well, you can always reinstall the theme if you break it. And if you break it real bad and can’t even get into wordpress, its still easy to fix.
But css is pretty easy to manipulate
https://www.w3schools.com/css/ is a great reference site
to change the font, you look for font in the css. font-family is the type of font used, font-size is size, just the word color is the color of the font.
https://www.w3schools.com/css/css_font.asp
really, don’t be afraid to mess with the css, its how we all start modifiying themes. Like I said, if something goes wrong, we can fix it.
Also, after editing and saving the css, when looking at your site, you may have to press ctrl+f5 on your keyboard toclear your browser cache and reload the page to see your changes.
Also, a great tool, if you use firefox browser, is firebug. It lets you point at parts of your site to see what css corresponds