stylish

Stylish is a user style manager that can change the appearance of web pages in a user's browser without changing their content by including user-supplied CSS style sheets with those supplied by the web site itself. The Stylish browser extension includes tools with which to write user styles, and can install user styles written by other Stylish users from a companion website. These user styles may be more or less selective, targeting just one web page, or all of the pages on a domain, or every page on the web.
Stylish was originally developed by Jason Barnabe as an XUL/XPCOM add-on for Mozilla Firefox. A Chrome extension followed in 2010 , which was released for Blink-based Opera 15 in 2013 and as a Firefox WebExtension in 2017. Similar extensions for Safari and for Presto-based Opera are distributed as 'Stylish' by other developers with Barnabe's approval.

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