changing font in fifteen theme

There are many tools available for this kind of CSS work, for example:
– Firefox with Firebug add-on: Help info on Firebug https://getfirebug.com/faq/
– Newer IE/Chrome, etc., have similar tools
– Alternately, try a dedicated CSS resource such as https://www.css-discuss.org/
– Also, consider creating a child theme otherwise all your customization will be overwritten when you next update your theme. Alternately, use Custom CSS Manager plugin: https://www.ads-software.com/plugins/custom-css-manager-plugin/