The most websites and also applications are very bright. So a dark application / website is something special.
I like a dark design. It creates a mystic atmosphere, which I really like.
But here it is important for what you use the application. The admin screen should have the same design lines like the whole application.
Dark applications are heavy. They maybe deppressed and makes a mood about comlexity. Bright design looks very light and easy to handle.
But IMO, dark designs can generate an extremly good atmosphere.
If you have an application which will be used for all the peoples out there, I wouldn't use a dark design. But it is for a special topic, for example gaming, movie etc., a dark design looks very good.
Also bright colors are even more effective on a dark background and you can create really stunning effects while playing with colors.
Do you maybe have some additional information about your target application?
A website which I love because of the dark design is GO! Experience design. It makes a mystic atmosphere and plays also with colors.