For now theme installation is supported for:

1. 🐦 Thunderbird 📦 download package. How to install for advanced users.

You can also use MenuLibre app to create a Thunderbird desktop entery:

• Install MenuLibre in the AppCenter.
• Download Thunderbird from the website and extract in a folder of your choice.
• Open MenuLibre, select Internet in the list on the left and then press the + button. Select Add Launcher.
• Go to Application details and enter the path to the folder of Thunderbird in Command.
• Add /thunderbird to the folder path.
• Press the icon to the left of New Launcher and select Browse Files.
• In Browse Files go to your Thunderbird folder /chrome/icons/default and select default256.png.
• Press New Launcher and type Thunderbird.
• Close the app and press the save button.
• You can now see the Thunderbird launcher in the Internet section of Slingshot and you can drag it to place it in Plank.

2. 🐦 Thunderbird 📦 Flatpak version. Install it using AppCenter.

Use this one line install script to install theme. Just copy the line to your terminal and press enter:

bash <(wget --quiet --output-document - "https://raw.githubusercontent.com/Zonnev/elementaryos-thunderbird-theme/main/install.sh")

Or install theme manually:

1. Install Thunderbird in AppCenter or type sudo apt install thunderbird in the Terminal.
2. Create the chrome folder in: ~/.var/app/org.mozilla.Thunderbird/.thunderbird/<profilefolder> if you installed in the AppCenter.
3. Create the chrome folder in: ~/.thunderbird/<profilefolder> if you installed in the Terminal.
4. Download the zip file and put the base.css in the chrome folder.
5. Look which window button layout you are using and put the userChrome.css from the corresponding folder in the chrome folder.
6. In Thunderbird 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.
7. Restart Thunderbird and the theme should be applied.