A polished Thunderbird theme for modern Ubuntu/Linux DEs

The Thunderbird email client interface may look fine on a Windows 10 PC, but when talking about Ubuntu, Gnome 3 based distros, things may be a little different.
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

If you like the updated Thunderbird look and want's to give it a try, that's how it's done:
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.