Modifications

For help with CSS, you really need to post a link to your site.

A browser tool like Firebug is the best way to work with CSS – using it to find the existing CSS – then COPY that CSS to the custom CSS option and making the change there.

A plugin is the easiest way to add Google fonts –

https://www.ads-software.com/plugins/wp-google-fonts/

Then use CSS to assign those fonts to any element of the page – again, Firebug will show you the CSS to use.

Good CSS resources –

https://getfirebug.com/css

https://www.w3schools.com/css/

https://developer.mozilla.org/en-US/learn/css