Sidebar length

The theme I use has one sidebar on the left. This extends the whole length of the page, regardless of content – even if there is a small amount in the sidebar, it continues to the bottom of the page with blank space.

I’ve added a right-hand sidebar, but this only continues until it runs out of content. I’d like it to continue down the page as the left-hand sidebar does, but can’t work out how. I’ve looked at the CSS, and used the Fx web developer toolbar to show the codes, blocks, etc, but I can’t crack this.

TIA for any advice on this.