Hello, I finished the elementary OS theme for Thunderbird.
If you install Thunderbird in AppCenter, create the chrome folder in: ~/.var/app/org.mozilla.Thunderbird/.thunderbird/<profilefolder> and put the userChrome.css file in there. Then enter the menu and select Settings, scroll all the way down till you see the Config Editor button. Press the button and search for toolkit.legacyUserProfileCustomizations.stylesheets and set it to true. Restart Thunderbird and the theme should be applied.

You can download the userChrome here:
userChrome.css.zip
Hello, I finished the elementary OS theme for Thunderbird.
If you install Thunderbird in AppCenter, create the

chromefolder in:~/.var/app/org.mozilla.Thunderbird/.thunderbird/<profilefolder>and put theuserChrome.cssfile in there. Then enter the menu and select Settings, scroll all the way down till you see the Config Editor button. Press the button and search fortoolkit.legacyUserProfileCustomizations.stylesheetsand set it totrue. Restart Thunderbird and the theme should be applied.You can download the userChrome here:
userChrome.css.zip