z-index

The z-index CSS property sets the z-order of a positioned element and its descendants or flex items.

For responsive

add -sm, -md, -lg or -xl to class name, for example:

<div class="zi10-md"></div>

.zi-1 {
    z-index: -1;
}
.zi10 {
    z-index: 10;
}
.zi20 {
    z-index: 20;
}
.zi50 {
    z-index: 50;
}
.zia {
    z-index: auto;
}