How to center DIV in a DIV?

This can be achieved by applying below CSS to the inner div:

#outer {
  width: 100%;
}

#inner {
    width: 50%;
    margin: 0px auto;
}

Of course, you don't have to set the width to 50%. Any width less than the containing div will work. The margin: 0px auto is what does the actual centering. It can be done in a different way as well.

#outer {
  width: 100%;
  text-align: center;
}

#inner {
  display: inline-block;
}

That makes the inner div into an inline element that can be centered with text-align.