Windows 10
Settings > Personalisation > Colours
Choose colour > Custom
Windows default and app mode set as 'Dark'.

In Thunderbird version 91.5.0
Menu app icon > Preferences > General
under 'Fonts & Languages' section
Click on 'Colours'
Choose the following:
'Text' is 'black' colour
'Background' is 'white' colour
Do Not select 'Use system colours'
Override the colours......'Never'.
click on 'OK'

Theme: Thunderbird default Light mode.
Start Thunderbird and select a folder - do not select any email - this is important.
Notice the Message Pane appears dark as if the Windows default app dark mode is controlling the Message Pane.
It will appear dark when selecting any folder providing no email has been selected.
This is the bug I'm reporting.

If folder has been previously opened and an email selected - upon folder reselection the last selected email opens, so you do not see the dark Message Pane.

When you select an email, the Thunderbird preference settings now control the Message Pane so white background and black text.

Expectation: The Message Pane should appear to use as the Thunderbird theme Light mode when it is empty.

This is the same if Thunderbird dark mode is selected, but is obviously not a real issue as it does not present a visual imbalance. Using same Preference settings, when email is selected, you see white background and black text.

Anje, does this still reproduce for you in version 102?
Someone on facebook is reporting something similar.

Testing using 'Troubleshoot mode' to ensure nothing is interfering.
Windows 10 OS
Using 102.4.2 - All OK, no sign of issue.
Updated to 102.5.0 and all still ok.