There's some controversy brewing in the comments about whether white-on-black or black-on-white is better. I thought of putting up a poll, and may do so yet if I can find a free service that doesn't use pop-ups. Anyway, thanks for the input.
My own feeling is that some colour other than white is a better background. White screens are by definition putting out maximum light and are hard on the eyes. In the end I may go with black text on some pale coloured background.
On the use of fonts; serif fonts were invented as a practical way to extend the life of block print in the old days of moveable lead type. If, for instance, the cross-arms of a lower case t started to wear out, a sans-serif t would look like an l. The serifs on the end, being relatively bulky, would wear out last and the vertical stroke would at least have two little dots on either side which the eye would fill in. So they're kind of redundant with computers.
Also, serif fonts always remind me of school books and newspapers. They seem somehow more academic and serious. Sans-serif can sometimes seem light and even trivial. It's all a matter of taste I guess.