fix dialog background colors (#4864)

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)

app/src/main/java/com/keylesspalace/tusky/AccountsInListFragment.kt

@@ -75,7 +75,7 @@ class AccountsInListFragment : DialogFragment() {
         viewModel.load(listId)
     }
 
-    override fun getTheme() = R.style.TuskyDialogOverlay
+    override fun getTheme() = R.style.TuskyDialogFragment
 
     override fun onStart() {
         super.onStart()