If Donald Knuth had designed Wikipedia
If you have a Wikipedia account, open your Preferences, go to the Appearance tab, and edit the Custom CSS for the Vector skin.
Paste style.css into the code editor, and save.
Alternatively, you can load the CSS using a browser plugin (such as Stylish for Firefox or Stylish for Chrome) for the domain https?://*.wikipedia.org.
For better alignment of mathematical equations, you can enable MathML or Mathjax in the Appearance tab.
There is no longer a link to the Preferences page on Wikipedia, so here's the URL.
Change the skin to something else, apply, and remove the Custom CSS from the Vector skin. Or, you could manually go to this URL while logged in:
https://en.wikipedia.org/w/index.php?title=Special:MyPage/vector.css&action=edit