Hi alessandrobruschi
As I can see that you set the white color for the trigger button so you don't see the icon now.
Kindly check this css rule in '/templates/t4_blank/local/css/custom.css' file on your site
@media (max-width: 991px) {
.site-default .t4-topbar {
display: none;
}
.site-default #t4-header #triggerButton {
right: auto;
top: auto;
position: relative;
z-index: 2;
color: #6c757d !important;
}
and add override the color property as above.