Just wanted to firstly say Webmaster and Admin are not the same things. You can have an admin based forum with webmaster areas 'n' such but they are not based around the same thing.
A dark theme is not unique.
Also to have a successful admin forum you do not need to be "unique". You need to provide good quality content, keep active and promote, promote, promote.
Obviously having some unique things is great but quality content is far more important. If you write articles and different things like that people will want to stay and read new ones and that also applies to other things like that.
A custom theme no matter how good will not make a community unique. It will make you look unique but that's a completely different area of uniqueness.