Add black as pronunciation guide color

[hftf]

MODAQ always renders all text in parentheses in the pronunciation guide color. Black is not an option for pronunciation guide color, but should be an option, because NAQT-style sets use an inverted convention for bracket punctuation: answerlines in parentheses and pronunciation guides in square brackets.

[alopezlago]

I had that at one point while implementing it and got rid of it, but I can bring it back

[alopezlago]

This should be back in the next version. I think I took it out because it won't play well with dark mode, but the reader can always change the color in that case.