Thunderbird 115 introduced a new look. I hate it. Not only does it no longer fit in with KDE Plasma, it lost functionality due to it's new integrated search bar in the title bar.
So I went on a mission to fix it.
To restore the regular title bar:
Go into Settings > General > Language & Appereance and uncheck the Hide system window titlebar
To remove search bar:
Toggle toolkit.legacyUserProfileCustomizations.stylesheets to true in Config Editor (bottom of Settings)
Find your profile directory via Help > Troubleshooting Information > Profile Directory (or about:profiles in same page)
This directory should have lots of files e.g. prefs.js and places.sqlite
Create directory chrome
Inside that folder, create a file named userChrome.css with the below content:
Haha there is always someone who dislikes all the space in today's modern design. I love the new look and it's why I'm using Thunderbird now as a new user. But I love that we have choice in the Linux community to alter things.
I was actually searching for a theme like this just a few minutes ago but didn't find anything, so thanks for sharing. Does this use the GTK 4/Libadwaita style?
It's sad that you hate it. It's good that you found a way to fight against change.
I will however admit that I didn't consider Thunderbird ac an alternative for my email management prior to v.115. Now I find it finally not ressembling a Windows 98 email client and really like it.