Center column styling: how can I remove margin or padding?

My goal is to have the background-color that I have on post headings span from margin to margin in the center column and then provide white space for the text of post between the left and right columns.

I believe I want to change my margins between the columns to zero and then have some padding in post content.

I can't figure out which setting to change in theme settings, though. Any thoughts?

My site is at:
http://depts.washington.edu/ophthweb/wordpress/

Thanks,

Bill