Recently I’ve implemented a styleswitcher with its js and CSS for a better usability, if you check the top right corner, you’ll find a link to a “high contrast” version of my website, easier to read and better accessibility.
Style sheets can be associated with documents using a list of link elements in the head. There are three different relationships external style sheets can have with the document: persistent, preferred, and alternate
Actually it was so well described, that I just had to implement it into the WordPress installation for a better reading. A list apart is the best resource for CSS and style techniques for websites: a lot to learn.