marinolab1 Not sure as to why specific parent CSS rule set isnt appending for your site although, it's working fine on my local install
Have instead added more precise CSS through grouping and the same reflects fine - Ref: Screenshot
CSS added in custom.css file :
.fa-youtube-square:before,.fa-twitter-square:before,.fa-facebook-square:before,.fa-instagram:before{color: #222 !important; }
You may add further CSS selectors in grouped CSS above for font awesome icons you wish to target for having specific color change to append.
You can always learn and make best use of firefox developer tool if using firefox and Chrome DevTools in case of google chrome which would allow you to examine, edit visually, and debug CSS extensively and with ease 🙂