A dark “theme” - really it just inverts the colors of everything except avatars, roles, and custom colors. Some custom colors have also been edited to make them more readable ingame.
This may slightly (?) reduce performance when navigating the site, as your browser will be inverting the entire page back and forth.
Works for both Firefox and Chrome, possibly more.
EDIT (7/6/18): Migrated to OpenUserCSS, options in progress.
A preview of options you can set in your extension