Centering blog in browser

My blog won’t center, but it seems that every answer I find in the forum is specific to a particular blog CSS.

Can you help me by looking at my blog to see why it centers in some browser windows and not others?

I understand that the problem is probably somewhere in margins and widths in the CSS and phps not matching up.

Do you see where I should focus my changes? (And also, if you don’t mind, so I can learn, how did you hone in on the particular place in the CSS or phps to find where the widths don’t match up?)

http://www.voltrek.com

Thanks.