I think selecting a light them at tools>addons&themes may do it. also check that in tools>general in the 'colors' box (under font info) does NOT have 'use system colors' ticked.

I'm getting this too, all my themes except for the default are in dark mode. These themes were not dark mode before the 102 update, and system colors isn't ticked

David is correct. Tools/Settings/Font & Colors/Colors, uncheck 'use system colors'. This will bring the body section back to regular viewing mode. Then Tools/Add-Ons & Themes, disable Dark Fox. Voila.

Still not the answer. System colors remains unchecked. Dark themes are not enabled. Thunderbird is still in dark mode.

Me too. EZilly fixed by disabling current theme then re-enabling. BOOM!

Disabling and re-enabling does not fix this for me, not sure about OP but none of these suggestions has resolved the issue yet for me.