dark theme option for evening work

[tomholub]

https://developer.android.com/guide/topics/ui/look-and-feel/darktheme

[DenBond7]

part of #714

[DenBond7]

for me https://designpub.ru/design-dark-theme-for-android-material-design-c7fb2e7a7ea3

[seisvelas]

User requested this

ref: https://mail.google.com/mail/u/human@flowcrypt.com/#inbox/FMfcgxwKkbqGfxdjknTqgrgGzNVwslCN

[martgil]

We received another user's request:

https://mail.google.com/mail/u/human@flowcrypt.com/#inbox/FMfcgzGpGnFpFgTbblMmMxMxqjVXxzTh

[DenBond7]

#1891 will close it