How to set columns height from top to bottom of the page?

hello everyone,
I just started a new site, first time using zc : http://www.hypeandunique.com/store/
I found a tons of good infos on the forum, but after a few days of searching and trying different methods, I just can't adjust the left and right columns so that they cover the entire height of the page from top to bottom.
I tried to modify the css stylesheet in different ways : for example

http://www.zen-cart.com/showthread.php?138030-extend-left-column-into-header&highlight=column+height

but I just can't move the columns without moving or affecting the content in between.

Any help would be great.

I did it with this

http://www.zen-cart.com/showthread.php?53712-Lavender-Template-Support-Thread&p=879099#post879099

based on instructions from

http://pmob.co.uk/temp/3colfluidtest_4.htm

hope this helps

Thanks for your reply, it looks like it could work for me.
But I don't know how to insert the code you suggested in tpl_main_page.php
I don't know much (almost nothing) about php.

Thanks again for you help