This change was made on purpose (see the release notes for 6.5.0) – the in-app theme setting was moved to "Display Options" (in the overflow menu in the reading view), and the background theme only applies to the reading view. This aligns with other reading apps (Kindle, Apple Books, etc.). If you want the full app to be dark, you'll need to set your device to dark mode.
That said, I agree that it's annoying that the app switches back and forth between light and dark when navigating around. Gospel Library has some similarities with other reading apps, but it also has differences in how it's used. And the Android app does still have an in-app switch to switch the full app to light or dark. You could send in a feature request under Help > Send Feedback in Gospel Library, to request that this be changed (the support team keeps track of common feature requests).
Samuel Bradshaw • If you desire to serve God, you are called to the work.