where can I change the background color?

[chenzhu]

I want to change the background color of left and right column. Can anyone help me?

[nemail]

Check the stylesheet for your site eg:
catalog/templates/aabox/stylesheet.css

You'll find all the stuff you need there - and lots more. Plus the links to any background images which have been used.
Keep a backup copy of the original, in case your enthusiasm gets the better of you!

Good luck!