Use Firebug to identify the particular elements you need to change. Josh has a good tutorial here: http://www.joshlobe.com/2011/10/using-mozilla-firebug-to-inspect-css-files/
Using Firebug, I see that the relevant <div> elements you need to adjust are #content-main, #sidebar1 and #sidebar2. Using the methods Josh outlines, you can play around the margin and padding settings of these items to get the effect you want. Try it — it works!