No single, up-to-date reference file seems to exist for the WordPress CSS style file (style.css). So, this document hopes to provide details on what configurable properties are available in that file.
The #content class is used for the main text on a page. It will include the topic headers and pre-more content on the first page as well as the topic content on each topic's page. This means that setting background-color, as an example, for #content will not change the background color of the header, footer, or the text-free sidebars.
I have no idea what the #page class is for. Modifying the background-color value for this class seems to have no effect on the display color of any of the pages.