It’s relatively simple as long as you remember that, on a design like this, what you add to the sidebar, you then have to take off the main post area.
Your sidebar (#sidebar) is currently 358px wide and your main post area (#content) is 550px – which gives you 935px to play with. So if you want a sidebar that is 450px wide, your content area will need to be reduced to 485px.
Step 1: Edit your theme’s style sheet at line 530:
#sidebar {
background-color:#E9EAEA;
float:left;
margin-left:24px;
padding:14px;
width:358px;
}
Increase the width setting appropriately.
Step 2: Move to line 525 in the stylesheet:
#content {
float:left;
padding-bottom:24px;
width:550px;
}
Decrease the width by the same amount as Step 1.
Hope that helps.