In https://github.com/tuskyapp/Tusky/pull/4851 I changed the theme of
`AccountsInListFragment`, which accidentally turned its background white
for the dark theme.
Additionally this fixes the color for the preference dialogs, which I
think have been incorrect since the Material3 redesign.
I also wondered if we should make dialogs darker for the black theme,
but looks like there is not much interest in that
https://chaos.social/deck/@ConnyDuck/113802937491059461
(Currently they are just the same as the dark theme)
