In Thunderbird Android in dark mode when the avatars are disabled, it is hardly visible which mails in the mail list are selected (they get very slightly brighter but this is really hard to see especially with a bit of sun on the screen). Please make the difference more pronounced.
When avatars are on, you see the checkmark on the avatars so there it is no problem.
Overall Thunderbird Android is great, these are just some small usability tweaks. Thank you