This is better for you in 8.0b5? See https://github.com/thunderbird/thunderbird-android/releases/tag/THUNDERBIRD_8_0b5

I have 8.05b installed on my phone and the highlighting of marked messages is still the same. I don't see a reference to a change in this in the New or Fixed notes for that version of Thunderbird Mobile nor in the branch comparison linked to from there.

OK. I see the shade you mention. It is probably from using Material 3.

But it's quite visible to me on a Galaxy S9+, plus each selected message has a big checkbox.

I have a Samsung Galaxy S21 Ultra 5G with Android 14 and OneUI 6.1. I have attached a screenshot showing the message list. The condition of each message is indicated by the wording of the subject line. As you can see the shades of gray are only subtly different. It would be very helpful if the highlight color was configurable.