An edit of http://freestyler.ws/style/118494/matu-s-dark-gitter. This has some fixes for the new site layout.

FYI, Gitter has added a built-in dark style which can be an alternative to this style since it's maintained by Gitter itself and it's okay (although it's a bit too light gray for my taste). You can select this theme in Gitter by clicking your profile picture in the upper right-hand corner and clicking on "Toggle Dark Theme".

Caveat: this style works better in WebKit-based browsers, such as Google Chrome or Safari, as it uses a WebKit-only property to darken some elements.