Try to use the following custom css at TemplateToaster -> General -> Css
.vm2-add_quantity_cart:hover, .vm2-remove_from_cart:hover{
background: url("
http://www.larteljee.be/components/com_virtuemart/assets/images/vm2-sprite.png") no-repeat scroll left top hsla(0, 0%, 0%, 0);
}
.vm2-add_quantity_cart:hover, .vm2-remove_from_cart:hover {
box-shadow: 0 0 0;
}
.vm2-add_quantity_cart:hover {
background-position: 0 0;
border: 0 none;
cursor: pointer;
height: 24px;
width: 24px;
}
.vm2-remove_from_cart:hover {
background-position: 0 -212px;
border: 0 none;
cursor: pointer;
display: inline-block;
height: 24px;
vertical-align: middle;
width: 24px;
}