Userstyle allows you to change the way websites look, so you can redesign sites you enjoy. Userstyle works by providing a file which your browser reads as cascading stylesheet (CSS) for websites you view with your browser. Those CSS rules can be made more important than page author rules, so yours stack on top of theirs.

Userstyle is not limited to Mozilla Firefox. It is a standard feature of all popular web browsers, including Internet Explorer (IE), Google Chrome (Chromium), Apple Safari, and Opera. Userstyle functionality is built into the cores of these web browsers; so, a browser built with WebKit or Mozilla will likely have the userstyle feature.

In a web browser like Firefox, where CSS is used to style the user interface, you may also employ userstyles to customize the look and behavior of your web browser.

Stylish is a Mozilla Firefox extension which provides an easier way to manage, write, and find userstyles for many websites. The tutorial will teach you what you need to know to write userstyles in Mozilla Firefox, using the Stylish Firefox extension.

Authoring userstyles is not easy. Doing so without knowledge of the web design concepts involved will be frustrating, at best. If you become confused while learning to userstyle refer to this section to find the definitive texts in these subjects.

Cascading Stylesheets: CSS: Cascading Stylesheets (CSS) is a method of defining rules for display behavior in a markup document like HTML web pages.

Don't Make Me Think Usable designs for normal people who want to use them. This book gives visual examples and easy explanations that help you undo what web designers are doing wrong. CSS: The Missing Manual Essential tutorials explaining new CSS properties and what you can do with them. Learn to easily make sophisticated interfaces, with the correct CSS for each job. The Web Designer's Idea Book Themes, trends, and styles; the best of over 20,000 websites. A showcase of some of the most creative website designs available, presented beautifully, for you to browse when you need inspiration for what to do next. Beginning HTML5 and CSS3 Stay ahead of the game with this comprehensive introduction to the next wave of web design technologies.

Document Object Model: DOM: Document Object Model (DOM) is a program memory structure into which a markup document is loaded by a browser. It's what allows CSS and JavaScript to easily influence the page, dynamically.

Markup Languages: HTML: Hypertext Markup Language (HTML) is the most common markup found in the Web. It defines the structure of a document telling software how to load the data into DOM, and provides named nodes which software may use to select pieces of the document data. XML: Extensible Markup Language (XML) is used to describe data structures, and is paired with XSLT (extensible stylesheet language transformations) which describes the XML tags with which you are describing data relationships. XHTML: Extensible Hypertext Markup Language (XHTML) is a type of HTML which uses XML to define HTML, forcing HTML to be written as HTML is supposed to be. XUL: Extensible User Interface Language (XUL) is an XML markup language which Mozilla Firefox uses to describe user interfaces.

Programming: JavaScript: JavaScript (JS) is a programming language used in web sites to create interactivity, and is also used to build Mozilla Firefox, and Firefox Extensions. You need to know that userstyles are affected by JavaScript code present on the styled page(s). JavaScript is used to modify page content, which could include CSS.