How to deal with cookies?

[DISLEX]

Let's say I want to have an option that can be set in the user cp. But I want the setting to be able to be stored in a cookie, so that whatever value is in the cookie takes precedence. Similar to styles. A user can have a default style set in their user cp, but if they change styles from the dropdown box instead of through user cp, that takes precedence and gets stored in the cookies (and thus can be different from computer to computer).

How can I go about doing this?