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 https://github.com/sidju/hired/commit/362c4a4a06b589f8372751adb17776dd7b0d6073 for relevant code change to clarify)