Note quirky colour behaviour around 16-color white and black

[sidju]

Since we now reset the color on white and black in 16-colour themes we should note that in the relevant code (which looks a bit strange now) and the theme declaration.

(
Possibly it would be good to create a way to more explicitly handle this in the theme itself, to provide a "colour" that is default and more explicitly use this in the theme.
)

(See 362c4a4 for relevant code change to clarify)