Material Design
The current UI looks very dated, since it's still using the widget style and some of the UI patterns from pre-Material Android. There is a fork of K-9 which makes the UI a bit more Material-y, but it's no longer maintained. Perhaps we could pick up those patches again and submit this upstream?
Repo: https://github.com/scoute-dich/K9-MailClient
Fdroid: https://f-droid.org/en/packages/com.fsck.k9.material/