以margin为中心的元素的负边距边框：0 auto;

I have a fixed-width page that I want to add a simple border to with the Border CSS command. However, I don't want this border to balloon the page and cause smaller screens to have a horizontal scrollbar. I'm not too great with CSS, but I know enough that I looked into using negative margins to offset the border's width since I had already done something similar to add borders to other elements that I don't want moving. But when I do so on my main container div, everything gets thrown off-center and smashed up to the left side of the page. I'm using the Blueprint CSS framework and I figured there was something in there that was messing with my margins, and I found the main container is applied a "Margin: 0 auto;" to center it on the page.I have a fixed-width page that I want to add a