css for the overlaying content

overlay-content.css

.vr-container .vr-overlay-content {
    position: absolute;
    z-index: 1; // positions the content over the viewer
    pointer-events: none; // this is important as it enables your interactions on the overlay to be detected by a-frame
}
.vr-overlay-content .action {
    pointer-events: initial; // enables clicks on the button, which was disabled by pointer-events: none
}