since some themes will have a non-white bg.

FWIW, if it ever were to really matter, you shouldn't rely on colors for this (or for anything, really), since some of us keep our browsers set to ignore the colors the page suggests, out of self-defense against all the garish choices misguided web designers make. (Black on white? Who on *earth* would choose *that* for display on a CRT? No, I don't want to be snowblind, thanks anyway.) For something like hiding a spoiler, it's not a big deal, of course.