Not a real issue because UI normally has white backgrounds on devices that aren't rooted and modified.
Tell us what should happen
Font color should contrast with UI notification color on devices that have changed the system UI's background color through means of a theme manager or mod (Xposed or theme flash through recovery)
Tell us what happens instead
In android's UI with a black background (not in k9), K9 notifications show the subject line font color that doesn't contrast with notification background color whereas the message contents does that just fine. The subject line remains in black font and cannot be read due to both colors of Android UI and subject line being the same.
K-9 Mail version:
5.113 f-droid build
Android version:
6.0.1
Account type (IMAP, POP3, WebDAV/Exchange):
IMAP personal

This is in v21/styles.xml
If someone from @TeamSubstratum knows the right thing to set it to so it's black on normal devices and something sensible on Substratum I'd be happy to test the standard case.
Currently it's
<item name="android:textColor">?android:textColorPrimaryInverse</item>
I'm not suggesting we change the theme. I'm suggesting we find a better constant to use to set the textColor. I thought Substratum would know that. But if the theme editors know that's fine too. It should be the colour that notification titles are set to.
Still happening on many dark themes.
This is now an issue on Galaxy S10 which has night mode on Android 9. While night mode is enabled you're unable to read the text of the k9 notification.
Same on Note 9
The problem (at least for me, S9+) happens only when I have multiple notifications, and only when collapsed:


