It's actually a combination of many factors, there is no 'should be all about any one particular thing' when it comes to GUI design. I could talk about GUI till the cows come home...I went to Uni, and graduated in the field of interactive multimedia development over 10 years ago. I'm too busy doing my own re-skin projects... to get into a long debate... I did find this though..
http://bokardo.com/principles-of-user-interface-design/
Personally, I prefer dark GUI designs for VST instruments because I primarily compose music at night in the dark (obviously not too dark to impair actual functionality). Light GUI's are fine for daytime use and in some cases absolutely necessary when bright sun shine is abound in your environment.