PopupMenuのbuttonにFontAwesomeを使うUserCSS
デフォルトのfont-familyにFontAwesomeを追加
popup-fa.css.popup-menu .button-container .button {
font-family: "Open Sans", Helvetica, Arial, "Hiragino Sans", sans-serif, "Font Awesome 5 Free", "Font Awesome 5 Brands";
font-size: 11px;
font-weight: 600;
display: inline-block;
text-align: center;
}