This isn't done by CSS specifically, it is done with the classic 9 celled table grid pattern and not much different to my
Links on Left-Hand Side tutorial.
Imagine a grid of 9 like so:
1 | 2 | 3
----------
4 | 5 | 6
----------
7 | 8 | 9
Numbers 1, 3, 7 and 9 house an image of the relevant corner
Numbers 2, 4, 6 and 8 house a background image repeated to show the edge
and finally number 5 houses the actual forum.
So if you follow my tutorial above, numbers 1,2,3,4 and the opening of 5 are placed in the header and the closing of 5, 6,7,8 and 9 are placed in the footer.