User Style Sheets are created by end users and shared with others by userstyles.org. User Style Sheets are mostly used by end users that have impaired vision.
As your site become popular (people start to create user style sheet for your site).
Avoid changing your style drastically / frequently, because the people that use "user style sheets" would have to update their style sheet to work with your new style, unless your new design would make it better for everyone.
How to use user style sheet with IE?
Go to "Internet Options" -> Accessibility -> General -> Format document using my stylesheet