Safari automatically toggles between dark and light background on reader mode based on the system dark mode setting.
Since Orion does switch its UI theme based on system dark mode setting, it should also do the same for reader mode.
Toggling dark mode on control centre would also make the reader mode use the dark background.
Safari seems to remember the bg colour on each mode
(e.g. If a user selected grey while on dark mode and white on light mode, it would switch to the corresponding bg colour on each mode), (i.e. there's no separate option for setting default bg on light/dark mode).
Their implementation looks straightforward enough to me (no need to delve into the settings) but a dedicated option in the settings would also work.
