However, when I install QGIS 2.12.0-1 via OSGeo4W there is only the option to choose the default theme.
Does anybody know whether there is another option I am missing to enable the dark theme?
According to the QGIS repository the Theme should be saved under
QGIS/ressources/themes. I am missing said
themes folder even after a re-install and a fresh install of QGIS 2.12 from OSGeo4W.
Nathan Woodrow confirmed it as a bug and is on it. In the meantime I copied the themes folder, like @Mapperz suggested, and everything looks great now.
Annnnnd it is fixed. That was fast!