Vim highlights the background of certain words (e.g. when doing spellchecking) in a way that I would like to change (because the background and the word itself are almost of the same color, which makes the whole thing illegible). How is it possible to determine which "background" (?) highlighting group (?) vim is using, so that it can be customized?
I did find an apparently similar-looking question, but its answer does not offer the simplest response (Ralf's answer below is simpler already), and more importantly, it doesn't cover the colors of the background (it displays the highlighting group of the text foreground).