Blank paper is white, so I will pick a contrasting colour (black) for the text I put on it.

A blank screen is black, so I will pick a contrasting colour (white) to right on it.

But as I am creating something to print out, I will write black text on white.

Most 70's and 80's computer games, and all film credits (OK - so you can think of a few exceptions), know that black was the most easy-on-the-eye colours when displayed on a screen. A white screen is simply dazzling.

Sensible applications that print to paper make it hard for those that see that screen text should be white on black.

Sensible websites copy this style.

Despite the fact that -

BLACK ON WHITE IS THE RIGHT COLOUR FOR TEXT ON A SCREEN.

Though sometimes I prefer subtler colours.