In Preferences, you would go to Guides & Pasteboard, in the Color section, and see what the Preview background color has been set to.
or maybe the interface has been set to Dark?
Go to Preferences > Interface tab, choose Light from Color Theme dropdown on Apearance section (upper part of the window)
preferences > interface > appearance > light - WHEW!!!
Type > font preview size > LARGE
My eyes were dying!!!