1) Add the libraries that you want to use in your theme's compony.info.yml file:

libraries:
  - compony/global
  - compony/font--slabo-27px--regular

(Only include the library definitions of the fonts that you need)

2) Use it in your CSS as follows:

font-family: 'slabo-27px--regular';

All the fonts are named using the BEM-naming structure.