This is the code to center an element that is positioned absolutely inside a relative parent.
.centered-xy { position: absolute; left: 50%; top: 50%; transform: translate(-50%,-50%); }
Center vertically only
.centered-y { position: absolute; top: 50%; transform: translateY(-50%); }
Center horizontally only
.centered-x { position: absolute; left: 50%; transform: translateX(-50%); }