I've added a new forum skin themed around Protohydroia Octherma, which also serves as a dark mode. To use it, go to the drop down in the bottom left of any forum page or go to My Controls -> Skin and Languages and select "Sagan 4 Octherma (Dark)".

The colors of some user groups have also been tweaked so that they are more clearly visible and readable in both dark and light color schemes.