Interesting thing a relative noted; I never noticed it because I leave transparency on. I'm sure some of you know this already, though: it seems that the 'transparency off' feature was improved (or, ruined, depending on your outlook) for Mojave: Rather than having the non-transparent dock be a bright and flat white (in light mode), Mojave has the dock assume the color scheme of the given wallpaper. Similarly, portions of windows which would otherwise be affected by enabled transparency will take that wallpaper's color scheme (albeit in a less pronounced manner). I posted three screenshots. The first shows my dock, without transparency, against a pink/blue desktop background; the second shows the translucent portion of my Messages window; and, the third shows that the dock (et al.) continues taking the color scheme of the desktop background, even with another window placed behind it.