Two interesting and rarely considered points in that guide:

① “Decrease the brightness and contrast of images a bit so that it looks comfortable to the eyes when it’s against a dark background.”

② “If we simply invert a dark shadow using light colors, then we get this funky thing […] Close elements should still be lighter and distant elements should still be darker – even in a dark UI.”

Oh yes, shadows are tricky. I got ① right for the dark theme of https://FreeSolitaire.win, but I never thought of ②. Good to know!