OverDrive rolled out an update last week that added a nice reading feature to its Libby iOS app. You can now use the app to check out a library ebook and read it in dark mode. This is a term used to describe when apps (including ebook apps) offer a color theme with reversed colors, or one based on a black screen.
The Kindle app has had this for a while, and so do a lot of reading apps. Kobo even built this into its ereaders. BTW, the Libby Android app also has this feature, and has had it for (I think) a few months.
Here's how the dark mode looks in the Libby app on my iPad.
Here's the changelog:
As well as preparatory support for notifications (coming very soon), iOS 13 compatibility improvements, audio and CarPlay crash fixes, and plenty of polish.