User-selectable CSS style
There may be a more elegant way to do this, but I would use the Switchtheme module, which provides a block that allows users to switch themes.

To make this work, you will have to create three separate themes, one for each option.  However, this is not as complicated as it sounds.  Just make the black-on-white and white-on-black themes subthemes of your main theme (the one with default CSS); then all you will have to do is set up the subthemes one time and add the CSS.