I did the same and it's works... Thank you...

I'm download Thunderbird 68.2.2 from official site, run application and install new icons to profile an all worked fine. Maybe you paste folder "icons" and file "userChrome.css" to another...

I installed Thunderbird beta 71 and I tryed, but doesn't work...

I use Arch Linux... I'm instaling Thunderbird beta 71... I'll try, if don't, I tell you...

Tested on Kde Neon with Thunderbird 60.9.0