When using z-index in your css try to choose a not too high value. Find the correct position in this list where your element needs to be placed.

You can use the z-index sass mixin to avoid a collision with the koala frontend z-indices. The mixin creates a z-index above or below the given koala layer. Optional you can set a offset to handle the z-indices between the given layers.

@import "kwf/z-index";

.cssClass {
    @include z-index(header, 'above', 2); // z-index: 52
    @include z-index(form-error, 'below'); // z-index: 29