CSS height style woes

Hello All

I am trying to help a friend with her Worpress blog layout. She's taken a shine to the primepress theme, however we are trying to make some chnages to it. Somethings seem to make little sense.

For instance we are trying to expand the border on the right hand side that surrounds the 'widgets'. She would like the border to run the full length of the page.

This at first seemed like a simple task, however neither CSS or Wordpress are up there as my strong points. Please can anyone help/advise where we need to be looking?

I have tried using height: 100% but this doesnt seem to work. The only way I can find to expand the box is by specifying the height in px. This of course over runs the rest of the page in the page content is shorter.

The wordpress site can be found here:

http://www.carlybond.co.uk/

Thanks in advance