Lately I switched back to KDE as I like to switch between desktop environments from time to time. One thing that always bothered me was how awful Thunderbird sticks out when using the default KDE Breeze theme. It is a beautiful theme but Thunderbird looks horrible with its GTK-ish icons. Especially because the folder icons are taken from the system theme:
Yesterday I saw a nice adjustment of the Monterail themes on Github (https://github.com/BDeliers/thunderbird-monterail) that gave me enough information to make at least the folder tree a bit more pleasing to the eye.
Thunderbird still does not fit in very well, but it looks tolerable. I have tried switching to KMail over and over again but keep coming back to Thunderbird as it is just a superior email client. And with these Breeze icons I can use it on KDE as well.
Code and usage information is on my Gitlab page: https://gitlab.com/raginggoblin/thunderbird-breeze