posts page

Hi
I am trying to style the layout of the posts page, but I don't know how. I have set a page as home page, and an other page as posts page. When I open the posts page, each post goes over the whole width of the page. I would like the width to be 75%, but I don't know how to change the layout of that particular page without effecting all pages. I have tried :

body.page-id-800 #middle {
width:75% !important;
}

but it does not work.

I have updated both Wordpress and Atahualpa to the latest version.

Would appreciate some help,
Harald