I recently created a dark based theme of Umbraco backoffice to extend the variations of not only being restricted to the regular installed appearance of Umbraco.
The package is open for collaboration. The structure and execution was kind of rushed. If anyone out there would like to collaborate on this package, get in touch with me.
If you're having problem not seeing the dark theme after installation, try logout and close down your active browser window if currently opened and open up a new one with and login to the backoffice once again. If this isn't helpful, try to raise the ClientDependency-version in the config-folder of your project.
Hopefully you find it enjoyable as much as I do! Give a like if so!