On: What’s New in Thunderbird 78

[commotionfever]

try set your GTK theme to a dark variant. eg start thunderbird with

    GTK_THEME=<theme>:dark

in my case it's

    GTK_THEME=Arc:dark

[aneutron → commotionfever]

Thanks ! That works wonderfully !

[omertuchfeld → aneutron]

I suggest you also install lxappearance to set the theme globally