That's the case with the Pop!_OS Linux distro and probably with other modern Linux Desktop Environments. So while Pop!_OS does look, works and feels great, the Thunderbird old look aproach somehow feels not fully integrated into the OS.
Until now, when I decided to try the spymastermatt/thunderbird-monterail themes and I'm very pleased about the result.
|Thunderbird default look in Ubuntu/Pop!_OS|
|Thunderbird stylish look in Ubuntu/Pop!_OS|
1. git clone or zip download the monerail theme files from here: https://github.com/spymastermatt/thunderbird-monterail
2. copy the resulting folder to your Thunderbird profile folder*
* in Linux, the fastest way to open the Thunderbird profile folder is to run this command
xdg-open /home/$USER/.thunderbird/*.default/in a shell console, while logged as a regular user, not root.
3. rename the newly copied folder to "chrome", without quotes
3. edit the userChrome.css (located inside the chrome folder), with your favorite text editor (I'm using Sublime) and replace the content with this one https://raw.githubusercontent.com/b247/b247/master/userChrome.css
Final words: the step 3 is not necessary required, the step 3 provided file is only to acquire the above look in a Pop!_OS distribution (should work in another DE as well). So you can keep the userChrome.css unmodified or tweak it yourself to gain the best visual integration with your DE.