That's weird. That doesn't happen for me in a test page here. I made a slideshow_blinds slideshow, by just pasting the code into a blank page with one centred div #contentWrapper setting the width to 960px in the page css:
border: 1px solid #999;
and adding the margin:auto to the dark.css for the slideshow_blinds stylesheet.
This keeps its full 960px width, I gave the div a 1px border so you can see the div.