CustomCSS Injector od Blizz3

Custom CSS Injector lets you write any CSS rules and apply them to any website you like. Change colours, backgrounds, resize and hide obtrusive elements. Anything you can do within the power of CSS!
Simply write the CSS you want to apply in the provided text field and click 'Save' (NO page refresh required!). Apply the CSS to the URL, Domain or Globally by using the radio buttons provided. There is also a whitelist/blacklist functionality to govern which websites you'd like to apply the CSS to.
Enjoy syntax highlighting and line numbers for code readability (Thanks to PrismJS!)