This works correctly:
display: block;
margin-left: auto;
margin-right: auto
else try this if the above only gives you horizontal centering:
.outerContainer {
position: relative;
}
.innerContainer {
width: 50px; //your image/element width here
height: 50px; //your image/element height here
overflow: auto;
margin: auto;
position: absolute;
top: 0; left: 0; bottom: 0; right: 0;
}