CSS question about subtheme

Let me preface by saying that I'm not a CSS expert, so if this seem naive please forgive.

I've made a fair amount of progress with a personal subtheme (basically a port of Black Piano to a 3 column layout based on OM).

As you can see you see in the screenshot, I'm trying to use a background image for the sidebars, but it won't extent down to the footer. I've tried using various 'height (max-height/auto/inherit/etc) ' settings in .region and #sidebar-* but nothing has worked.

Here's the CSS for the sidebar:

#sidebar-first { background: url("images/sidebar.png") repeat-y scroll 0 0 transparent;
                 color: #999999;
                 min-height: 450px;
                 
} 

The 'content' (#main) section starts with a min-height of 450px also. Its when you publishing content and extending this section that it becomes a problem.

If you would like more info please let me know.

Thx in advance!