Using the theme folder file with the extension .info

E.g., sites/all/themes/bootstrap_subtheme

bootstrap_subtheme.info

considering, sites/all/libraries is the path for the fontawesome library:

; FontAwesome 4 CSS
; --------------
stylesheets[all][] = ../../libraries/fontawesome/css/font-awesome.min.css


; FontAwesome 5 CSS
; --------------
stylesheets[all][] = ../../libraries/fontawesome_v5/css/brands.min.css
stylesheets[all][] = ../../libraries/fontawesome_v5/css/all.min.css

Be sure to verify, that @font-face property is enabled - signifgicatly reduces load upon a page speed performance

• https://tools.pingdom.com
• https://pagespeed.web.dev/ (former https://developers.google.com/speed/pagespeed/insights)
• https://www.webpagetest.org
• https://gtmetrix.com

@font-face {
  font-family: 'FontAwesome';
  src: url('../fonts/fontawesome-webfont.eot?v=4.7.0');
  src: url('../fonts/fontawesome-webfont.eot?#iefix&v=4.7.0') format('embedded-opentype'), url('../fonts/fontawesome-webfont.woff2?v=4.7.0') format('woff2'), url('../fonts/fontawesome-webfont.woff?v=4.7.0') format('woff'), url('../fonts/fontawesome-webfont.ttf?v=4.7.0') format('truetype'), url('../fonts/fontawesome-webfont.svg?v=4.7.0#fontawesomeregular') format('svg');
  font-weight: normal;
  font-style: normal;
}