Hmmm... bit tougher if only want it on that page... as the elements you need to modify are outside the component body and hence won't be affected by a page class suffix.
Three steps:
1.) Remove all the css you added earlier (the -15px margins etc)
2.) Try and get the following css above the element we need to style.#main-content {
background: #000000 !important;
}
The best way to do that would be via REMOVED but it probably won't work in j1.5 even in legacy mode. If you can convert it to 1.5 native, great, otherwise, look for something similar in the JED that works for j1.5.
One last option is to stick the css right into the code. This will break xhtml validation but who cares, really.
Turn off code cleanup in your WYSIWYG Editor, or turn off the WYSIWYG Editor completely and then paste the following into the BANNER module position.<style type="text/css">
#main-content {
background: #000000 !important;
}
</style>
Make sure this module is only published on the page in question.
Let me know if that works.