Nevermind, there is a typo on the code above. For those wondering, this is the right code:
#mobileMenuLink a:before {
content: "";
background-image: url(https://image.flaticon.com/icons/svg/1279/1279537.svg);
background-repeat: no-repeat;
background-size: contain;
background-position: center center;
width: 30px;
height: 30px;
display: inline-block;
visibility: visible;
}
#mobileMenuLink a {
visibility: hidden;
}