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

libraries:
  - compony/global
  - compony/font--roboto-condensed--bold-italic
  - compony/font--roboto-condensed--bold
  - compony/font--roboto-condensed--italic
  - compony/font--roboto-condensed--light-italic
  - compony/font--roboto-condensed--light
  - compony/font--roboto-condensed--regular

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

2) Use it in your CSS as follows:

font-family: 'roboto-condensed--bold-italic';

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