How to set font-display to swap for local fonts

Our site, grownbyyou.com, repeatedly gets Google PageSpeed diagnostic warnings to “Ensure text remains visible during webfont load” because font-display: swap isn’t functioning for 3 theme-controlled, locally-hosted font families, FontAwesome, Icomoon, and hbicons, and page processing stops to load and check the fonts before painting the page. I’ve tried verious ways of adding CSS code in Highend>Customize>CSS for these 3 font families, but error still continues. Please coach me through how to correct this problem so that each of our pages’ processing and display isn’t delayed ~250 milliseconds due to these fonts. I’ve implemented a plugin to fix this problem for google-supplied online fonts, but it doesn’t patch local fonts. Thanks.