This extension reskins YouTube into a darker and simplified version.
Every page including the video, browse, subscription, upload, and search pages is updated to match the style. The navigation fonts and buttons have also been slightly changed for better readability.
Opera and Firefox versions are here (including screenshots since Chrome won’t allow me to upload them for some reason):
Official Chrome extension: https://chrome.google.com/extensions/detail/gkkdoohkeppiioiikkkekdbnbkobempe
Opera userstyle: http://userstyles.org/styles/40455
Firefox Greasemonkey Script: http://userscripts.org/scripts/show/91359.