Would it be possible to add a unique id to the body of the pages using the sitescheme (profile, entry non-customised pages, etc.)?

I like black text on white background, and none of the siteschemes really suit me in that regard, so I'd like to be able to customise a stylescheme via the Stylish extension on Firefox. But as there is no reliable way of determining whether you're on a sitescheme page or not, I run the risk of my customisations bleeding on other pages.

Thanks!