User-selectable CSS style I have three CSS versions (default CSS, black on white and white on black for visually impaired users) of my drupal-7 website to be selected by users if they need to.

And I just have no idea how to do this with drupal... I would like to add a drop-down list in the footer to change the CSS used. Then, I imagine I would have to store it in a session variable or a cookie and change the CSS according to this value.
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.