Somehow I manage to turn the outer BG to white (from the original dark gray). I have tried changing the preview mode (Pixel & Overprint) and to change it through the preferences menu, but it didn't work. How can I fix it?
Things which can alter the pasteboard color....
View Menu > Outline, Overprint Preview, Hide/Show Artboards, Hide/Show Transparency Grid
Preferences > User Interface
Most probably you've accidentally hit Alt+Shift+Ctrl+Y which is the shortcut for Overprint Preview (View -> Overprint preview).