There are multiple ways of doing this.
1. Using Positions : We can use position relative and absolute.
How?
Make the window or the wrapper container as relative and set width and height as 100%.
Make the inner div(which we need to place in the center) position absolute and all values as 0. Suppose the inner div is having id as 'centered-div'. Then our css would be.
#centered-div {
position: absolute;
...
