using page/post id to specify per-page css in 2.5

I’m about to upgrade a site from WordPress 2.3 to the latest version. I’ve previously been employing the page id in order to have page specific css, e.g., pages with either 0, 1 or 2 sidebars.

For example, if the url is http://site.com/page1, the page is styled with a combination of a template and css to specify the width of primary:

#page1 #primary {margin-left: 0;
width: 890px;
}

for example.

I’ve read in some other posts here that WordPress 2.5 no longer uses this, and now the post id is used instead, and I’m not sure how to specify this in css. I can usually fumble my way through, but I’m a bit stumped here. Can anyone offer me some advice? Thanks!