Microsoft bought popular third-party keyboard Swiftkey four years ago, and the app is still going strong. It refreshed its branding earlier this year, and started work on supporting Android 10's dark theme. Now it looks like the latest beta version supports automatically changing the keyboard style to match the look of the system theme.
Frequent TV watchers or ardent fans of specific series are likely familiar with SeriesGuide, which makes it much easier to follow your favorite shows, get notifications for new episodes, and even sync your lists across devices. A new version of the app (v53) is rolling out now, and trickling light/dark theme switching down from an in-app purchase to a standard feature, and bringing support for system-level theme switching on supported devices, among other smaller changes.