How to alter site-wrapper width

I have tried to remove the border around the slider with success. However, I try to reduce the witdh of the site-wrapper in order to align the menu on top without inserting padding.
I try to declare site-wrapper in the style.css in the theme child directory. However when I look at the HTML-log in Firebug I see that my style.css is loaded before gallery.css. This means that I have to alter gallery.css in order to change the width. Is this correct, or have I missed anything?