Black on blue/purple (what color is that?) and viceversa are probably fashionable but not the most readable combinations. For sure they are a way to force reade

Black on blue/purple (what color is that?) and viceversa are probably fashionable but not the most readable combinations. For sure they are a way to force readers to put extra effort into reading so maybe it's a deliberate choice: whoever gets the point is really interested. Survivor bias.