We had a chance to go hands-on with some Samsung test devices running the company's new One UI - the latest in a long line of TouchWiz rebrands - and the feature likely to generate the most attention is almost certainly the new "Night theme" option. Basically, night theme sets nearly every stock app, along with the launcher and lockscreen, to an all-blacked-out look - something Android enthusiasts have been asking of device makers for years. My initial take is that Samsung's done this pretty well, and essentially every stock app on the test device had a dark theme. Here's a quick look at some of those apps in a gallery.
Some apps, like Smart Switch and the theme store, weren't yet sporting dark modes, but I imagine that's just a result of this still being a very early beta, and those apps likely being handled by other development teams outside the core UX group. If there's one thing that can be said about Samsung, it's that it's capable of throwing all its weight and resources behind an initiative, so I'm sure essentially every Samsung-published app will get with the program on this.
As to whether third party developers will start taking dark themes seriously? We can't know for sure, but with Samsung being the single most important player in the high-end Android smartphone space, this is a company that we know can shape the priorities of its partners - and competitors. So, if Samsung phones get a dark theme, you can probably expect other manufacturers to follow suit, at least eventually. We all know Google has been slow to get this rolling as part of the stock Android experience, so perhaps Samsung will be the kick in the butt that finally makes it a priority.