Dynamically Allowing a User to Change the BG Color &Font Color for accessibility

Hi,

I am new to wordpress and PHP, so please be gentle. I am putting a website together for a local charity who offer support to the visually impaired.

They have requested 2 dynamic functions that should form part of the site and im not sure what the best way is to go about it.

1) The user must be able to increase / decrease the text size to suit there own needs.

2) The user must be able to change the background color and font color to suit their needs i.e. White Text on Black Background, or Black Text on Yellow Background.

A good example of site that has this functionality is http://www.herib.co.uk/

If anyone could provide any insight into how this is best achieved i would be very grateful.

Thanks