Thunderbird 115.4.1 (64 bits) problem : dark theme cannot be deactivated

[pboe]

I am on Linux Xubuntu 22.04.3 LTS and since a few days, Thunderbird 115.4.1 (64 bits) has the problem that dark theme cannot be deactivated. I activate the white theme, but it remains in the dark one.
Please help me !
Philippe