It seems like Google's Pixel Launcher may actually get a proper dark theme in the near future. In March, a user asked Google to include a toggle to choose between dark and light themes in the Pixel Launcher, and it seems like Google complied. This request was made via the Google Issue Tracker, and Google has now marked the item as fixed, and said that the Dark theme support has been added to quick settings and launcher under Settings -> Display -> Device Theme. The company also said that this change will be made available in an upcoming Android build, so chances are that Pixel users will get it in the next developer build of Android P.
At the moment, Pixel Launcher basically automatically chooses between Dark and Light themes based on your wallpaper. If your wallpaper is dark, you'll get a dark theme, sort of, and that applies to your folder icons, quick settings, and app drawer. This change will allow you to toggle between light and dark themes without having to worry about your wallpaper in the process, you'll be able to select whichever wallpaper you want and have whichever theme running alongside it. Google may still leave an option for the Pixel Launcher to choose a theme based on your wallpaper, though, if that's what you prefer, though that has not been confirmed just yet. This change will probably be active in the next dev build, based on Google's comments, and it will be making way to the final build of Android P aka Android 9.0, once the update becomes official with the Pixel 3 and Pixel 3 XL later this year.
A ton of third-party launcher have been rocking dark themes for a long time now, an option to choose a dark theme, that is, and the same goes for a number of other first-party launchers that OEMs include along with their Android skins, like OxygenOS, for example. The inclusion of a dark theme in the Pixel Launcher has been one of the most requested features when it comes to Google Pixel users, and it will finally make its way to the Pixel Launcher.