Last year, both Google and Apple added dark theming options into their mobile operating systems, with many system apps supporting the feature. Some apps took longer than others, though, like Google's own search app. We reported that its dark theme was available through the Play Store Beta program earlier this year, and now the feature is rolling out to all users on Android 10.
Ever since the company's short-lived relationship with Cyanogen, Inc. collapsed, OnePlus has shipped a custom build of Android called OxygenOS on its phones. It's generally regarded as one of the best manufacturer skins, since it adds useful features on top of stock Android, while not modifying the core Android experience (which can sometimes break apps). However, there is one aspect of OxygenOS that is a clear regression compared to stock Android: dark mode.
The ability to change the appearance of the Android system has always been missing from stock Android, with the exception of the Overlay Manager Service, which is only configurable through the use of third-party tools like Substratum. This somewhat changed with the Pixel 2, where the quick settings dropdown and launcher would switch between light and dark themes based on the user's wallpaper.
During CyanogenMod's heyday, one of the ROM's biggest features was the Theme Engine. You could download any number of themes from the Play Store to change up the entire system's look. However, the feature didn't make an appearance in CyanogenMod 14 (based on Android 7.0 Nougat), and there hasn't been much talk of working on it since the project reorganized into LineageOS.