Stylish is a free Mozilla

Mozilla

Mozilla is a term used in a number of ways in relation to the Mozilla.org project and the Mozilla Foundation, their defunct commercial predecessor Netscape Communications Corporation, and their related application software....

 extension that allows for the manipulation of web pages and XUL application user interfaces through the use of Cascading Style Sheets

Cascading Style Sheets

Cascading Style Sheets is a style sheet language used to describe the presentation semantics of a document written in a markup language...

. These custom "user styles" are created by the user, or installed from an external archive like userstyles.org. It is available for Mozilla Firefox, Mozilla Thunderbird

Mozilla Thunderbird

Mozilla Thunderbird is a free, open source, cross-platform e-mail and news client developed by the Mozilla Foundation. The project strategy is modeled after Mozilla Firefox, a project aimed at creating a web browser...

, Google Chrome

Google Chrome

Google Chrome is a web browser developed by Google that uses the WebKit layout engine. It was first released as a beta version for Microsoft Windows on September 2, 2008, and the public stable release was on December 11, 2008. The name is derived from the graphical user interface frame, or...

, and Flock

Flock (web browser)

Flock was a web browser that specialized in providing social networking and Web 2.0 facilities built into its user interface.Earlier versions of Flock used the Gecko HTML rendering engine by Mozilla....

.

Technical details

User styles are CSS style sheets designed to alter the appearance of one, some, or all sites, or of the browser itself, and are applied only to the targets specified. Individual user styles can be enabled or disabled in Firefox without having to restart the browser.

User styles are added to the CSS rules provided by the site, but can also override the site's styling (often requiring the !important keyword for each replacement rule). The most common uses are ad-blocking, applying a new color scheme, and eliminating unwanted page elements.

There are three classes of user styles:

App styles

App styles don't change the appearance of Web sites, but rather modify the web browser's user interface. This is similar to the userChrome.css CSS file used by Firefox and Mozilla-based browsers.

Comparison to Greasemonkey

Stylish is often compared to Greasemonkey

Greasemonkey

Greasemonkey is a Mozilla Firefox extension that allows users to install scripts that make on-the-fly changes to HTML web page content on the DOMContentLoaded event, which happens immediately after it is loaded in the browser .As Greasemonkey scripts are persistent, the changes made to the web...

, another Mozilla extension that allows client-side manipulation of web pages.
Greasemonkey's user scripts are essentially dynamically inserted JavaScript

JavaScript

JavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles....

that can alter a page's appearance or functionality, while Stylish uses CSS, which can only alter appearance.

The style repository at userstyles.org allows you to add custom made stylesheets as Greasemonkey scripts, and many stylish scripts have the option of being added to Greasemonkey.

Example

Below is an example of an Stylish user style being applied to a web page.