Thunderbird monterail is a free theme for thunderbird email client.It uses the userChrome.css file to change the look of Thunderbird. Currently it comes with five theme variants. The variants are System, Light: Light, Dark, Full Dark and Monterail. It uses Font-Awesome svg icons and EncodeSans Narrow font.
Install Thunderbird monterail theme on Thunderbird
Download the latest Thunderbird monterail theme from the theme download page.
Here the downloaded file name is thunderbird-monterail-master.zip. Right click on it and select Extract here to unzip it. Rename the extracted thunderbird-monterail-master folder to chrome.
Go to your Ubuntu Home Folder and press Ctrl + H key to show all hidden files and folders.
Now open the .thunderbird folder. Then open profile config folder. It looks like [random letters and numbers].default.
Now copy the chrome folder to this folder. Inside the chrome folder open userChrome.css file with any Text editor.
Then find below line in the file
and replace system.css with name of the theme variant. Like
And save the file and close it. Now open Thunderbird email client to check your theme.
Other themes css files
dark.css fulldark.css light.css