Override a default favicon with an image of your choice. In this case, I am replacing the .fa-folder-open icon.
/* Hide the original font icon */
.tb-menu-icon .fa-folder-open {
font-size: 0 !important;
position: relative;
}
/* Replace with Box logo */
.tb-menu-icon .fa-folder-open::before {
content: "";
display: inline-block;
width: 22px;
height: 22px;
background-image: url("https://cdn.freebiesupply.com/logos/large/2x/box-39-logo-png-transparent.png");
background-size: contain;
background-repeat: no-repeat;
background-position: center;
}