Thanks for your rating. It's great to hear that you like the add-on. 😀

The feature you requested (dark theme in options page) is tracked in https://github.com/rugk/offline-qr-code/issues/190 and may indeed be added in the future.
A dark theme for the popup is already implemented for the next release: https://github.com/rugk/offline-qr-code/issues/186