Accesibility standards and cookies

I'm just about to start developing my first Drupal-driven site.

The site is intended for use by some visitors with different sight difficulties, and I wanted to be able to give visitors a choice of font size/style and colours, contrasts etc, as well as perhaps different page layouts. which they can choose (probably by selecting a from a range of different sub-themes) and which can be saved as a preference for the next time they visit the site. I have not yet coded "in anger" with Drupal, but can anybody suggest how this might be achieved?

Regards,

Martyn