Hi David75, and welcome to WebmasterWorld :),
What a very interesting question.
Am I right in that you want this applied to every page the user visits - not just pages you have created yourself?
If so, then as Lucy says it is a little beyond css sole, but my first thought is that it looks "do-able". If it is, I think it requires a combination of css, adjusting user browser settings and I wonder if the accessibility guru's will have some techniques to help as well - so I've asked the browser and accessibility guru's to drop by to make suggestions.