1. In Drupal 8 open directory is /themes/custom/THEMENAME_sub/css/
2. In Drupal 7 open directory \sites\all\themes\THEMENAME\THEMENAME_sub\css\
3. Original __custom.css should be renamed to custom.css and after this drupal cache should be cleared
After this you need to add extra CSS styles to custom.css file for example:
@font-face |LF|