Keep in mind that if you want to use a special font, you’ll need to also enqueue it in the theme. If you just specify it in the CSS, that’ll only let folks see it if they happen to have that font installed on their computer, which limits you to a very narrow set of fonts, like Arial, Georgia, Verdana, etc.
If your font is a Google Font you can use a Google Font plugin to enqueue it in your theme:
https://www.ads-software.com/plugins/search.php?q=google+font
You’d then target it in your CSS using the method ashiquzzaman mentioned above.
Don’t edit the theme files directly, otherwise your changes will be overwritten every time the theme is updated to the latest version.
An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.
As alternatives, you could either install a standalone custom CSS plugin, or create a child theme.