Imo its simply the darkness that text gives to the game. Imagine playing rogue with grey text on a white background. It doesn't work, does it?
The black background just darkens the whole mood of the game far more than any graphical prettying up can do. Prettying up just puts someone else's imagination into play, and it'll often be less intense than your own. After all, what's actually more worrying - seeing a picture of a dragon someone else has thought up and knowing its going to kill you, or seeing a little "D" appear in your character's vicinity and thinking "Oh shit"?