ah, the ol' double duty. I think that happens to everyone.
What you need requires quite a bit of coding. i'm not sure I can sit down and do the whole thing, but I found this and it is probably the best example with clear explanations as to what is going on and what you would need to do.
This guy is good. I actually have his book as well. I learned a lot from his site in my early years of coding. For your scenario, these are the steps you would need to take.
1. all of your pages must check for the existence of your cookie. if it exists, use the values from that cookie. If it doesn't, then load the default style.
2. when a user selects a different style, you will need to check for the existence of your cookie (you will always have to do this), if it exists, delete it and write the new cookie, if it doesn't then just write the cookie.
your cookie could be set up as follows: myStyle=blue; expires=Thu, 2 Aug 2001 20:47:11 UTC; path=/
what you would change (when a new style is selected) each time is the color after the = sign. This will all be js and you may have to piece bits together such as this cookie script with a script that changes page styles, but it can be done (even by a novice to js - it will just take some time). The alternative is to hire someone to do this for you (unless someone else has the time to whip up a script for you here). hope this helps