I'm editing Bluemarine now and using the Firefox Web Developer kit. I'm not finding any place where the background images for much of the site is being set. I try one after another background color style and come up with no change as I'm setting each one to black background. Maybe I should move everything to my pc and use Dreamweaver to search each and every file, telling it to seach the code for background-color.
No idea about the developer toolkit, but your main problem is discovering which style has the highest priority. It could be a bit weird. As a quick hack - you can create your own class or id and make sure that it appears only on that particular page. Then style it with high specificity. NOTICE: This is a quick hack, it is ugly, it might quickly lead to serious brain damage, let alone mess.
About your original question - how to change the backround on a particular page - if you want to change the style of a particular set of pages, you might have a success with the sections module.
Alternatively, there used to be a tip in the handbook, giving code example of how to do it within the theme template, without the sections module. Not sure where to pint you to though. But the idea is simple: in the beginning of the tpl file do conditional loading of templates - if, switch, whatever you fancy.
An advice, if you want to save yourself some brain power, spend some time and split drupal.css into logical components and get a feeling what is used there. It looks scary, but it is fairly straight forward, and is well documented inside the file. The themes, tend to be messier, due to beautification hacks and are not documented to that extent.
Apropos, I like shooting in the dark, just don't tell the kittens
Cheers, Vlado