Removing login text and showing account icon on mobile

Site URL: https://www.santucoffee.com/

Hello! I'm a bit stuck with some custom code and hoped someone might be able to help. I've managed to change the login text on desktop to an account icon but now on mobile I can see both the text and the icon and it all looks a bit odd. Is there a way to remove the text and move the account icon into the header on mobile? Can anyone help me with this please? Thanks very much :) 

Add to Design > Custom CSS

/* hide login text mobile */
@media screen and (max-width:767px) {
span.unauth {
    color: transparent !important;
}
}

A very late reply but thank you so much for your help @tuanphan