The user style sheet that B&N uses, has a higher priority than any other CSS elements (and includes rules on the body element). So while eventually you may figure a way around this, you are only one update away from it not working the way you expect.
However, on any B&N reader if you want your styling to be used, then you need to go set 'Publisher Defaults' to 'On', which will disable the user style sheet, and get it to pay attention to your styling.
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