Mantra was not created with dark backgrounds in mind, it doesn’t have built-in options for changing absolutely all colours and we never got around to adding such options.
But it CAN be completely customized with custom CSS. You just need to use a developer tool like Firebug to inspect the content of the website and figure out which areas you need to change colours to.
Or tell us a bit more clearly (with page name and area content/description) what you are trying to change.