"i noticed they won't scroll down to the actual window size"
I thought you meant by this that your centerbox did not stretch all the way to the footer like a side column does. If you really meant that the box sides didn't go all the way to the bottom of the box, that is a completely different problem, and first trading is on the right track.
If you make an image for the heading, one for the bottom of the box and one for the sides
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These will work for a box that has a wrapper or container div, a heading div and a content div.
Attach the top to the heading, the bottom to the content and the middle to the container with repeat-y so it repeats vertically for the full height of the box. The top and bottom want to be no-repeat.
The centerboxes typically have only a container div and a heading div, so you can't use three images. For this, make a top image attached to the heading and a middle/bottom image attached to the container. Make the sides of the middle taller than any box you will ever have.
Code:
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