For different color sets, also add one of these styles:
→ http://freestyler.ws/style/57286/wikipedia-dark-blue-simple
→ http://freestyler.ws/style/58503/wikipedia-dark-gold-simple
→ http://freestyler.ws/style/57348/wikipedia-blue
→ http://freestyler.ws/style/56882/wikipedia-dark-green-lustrous

Works on most Wikimedia sites, such as wikipedia.org, wikimedia.org, wiktionary.org, wikiquote.org, wikibooks.org, wikisource.org, wikinews.org, wikiversity.org and mediawiki.org.

Changelog:

2015-01-04 - fixed Table of Contents on German Wikipedia.
2014-09-27 - minor fix for "written script" selection menu on Chinese and Serbian sub-wikis.
2013-11-13 - references now collapse by default (if JavaScript is enabled); to make them expand, just hold your mouse over them for about a second; other minor fixes.
2013-09-19 - fixed the new "VisualEditor", again.
2013-09-05 - fixed position of left menu.
2013-08-05 - fixed the new "VisualEditor".
2013-07-28 - fixed the new Table of Contents.
2013-07-23 - fixed links to sister-projects, thank you for reporting.
2013-07-07 - fixed the new language selection popup.
2013-05-31 - added wikivoyage.org domain.
2013-05-14 - added URLs starting with "http://www.baka-tsuki.org/project/".
2013-05-07 - update because of changes in how Firefox (and soon Chrome) render multiple columns; added columns to languages menu on Opera as well.
2013-04-04 - fixed a Chrome specific issue with .vectorTabs menu overflowing.
2013-02-26 - fixed the crazy Wikipedia blunder they've made today with their updates; added wikidata.org domain.
2013-02-11 - fixed mobile subdomain; fixed a couple more unordered lists.
2013-01-28 - Support for registered users.
2013-01-01 - New Year present - introduced several more menus at top, including print! ;) Languages and navigation menus now represented with images. Name changed/shortened. Enjoy!
(old version http://freestyler.ws/style/56697/wikipedia-layout-enhanced-textual-menus, but will not be maintained)
(older items truncated)

Based on http://freestyler.ws/style/31157/modernwiki-night (version from 2012-03-12) by vetinari@userstyles. Original license: http://creativecommons.org/licenses/by-sa/3.0/