FluentBird is a userChrome.css theme for Mozilla Thunderbird, that implemenets Windows 11 Fluent Design and Mica transparency materials.

Created by Danny King - www.dannyking.co.uk

Released under the MIT License

• Implements Fluent Design styling in Thunderbird
• Supports both dark and light modes (system theme)
• Enhances and highlights Mica transparency on Windows 11 systems

This will likely be the last beta release, unless more issues are raised, with 1.0 release to occur alongside Thunderbird 141.

• Improves the "no messages" envolope graphic in dark mode.
• Fixes the subject line clashing with the star / tags icons in card view.

• Adds Fluent icons and correct button styling for the "Reply List" button
• Fixes button hover colour in Dark Mode
• Fixes subject line colour on card view in Dark Mode

FluentBird uses Fluent Design icons provided by Microsoft, licensed under the MIT License. Copyright (c) Microsoft Corporation.

1. Enable userChrome.css

   • Open Thunderbird
   • Go to Settings > General
   • Click Config Editor… under Advanced
   • Search for toolkit.legacyUserProfileCustomizations.stylesheets
   • Set it to true
   • Restart Thunderbird

2. Enable the Mica flags in Thunderbird’s (Firefox-based) advanced configuration:

   • Open Thunderbird
   • Go to Settings > General
   • Scroll down and click Config Editor… under Advanced
   • In the search bar, type widget.windows.mica and set it to true
   • Search for widget.windows.mica.popups and set it to 2

3. Make sure no other theme is selected in Thunderbird settings. Do not select Light, Dark, or any theme from the Thunderbird extension store. The theme must be set to "System Theme" or simply unselected.

4. Copy the included files to the chrome folder in your Thunderbird profile directory:

   • Click on Help > Troubleshooting information
   • Scroll down to "Profile Folder" under "Application Basics" and click "Open Folder"
   • Create a new folder named chrome inside your profile folder, if it does not already exist
   • Place the userChrome.css file and all folders into the 'chrome' directory

5. Restart Thunderbird.

• This is a beta release. Expect bugs and incomplete styling.
• Untested on Mac and Linux (should work, but without Mica).
• Report issues via the GitHub repository.

Also, note that some areas of ThunderBird are rendered outside of the influence of userChrome.css in a "Shadow DOM" - as such, it is not possible to fully theme all elements of Thunderbird.

• The mail section is fully themed, as is the general window chrome. The Contacts, Calendar, Tasks and Chat functions have limited themeing in some areas.
• It isn't possible to style the new message / reply message window, or any other pop-up windows.
• As they are entirely contained within a Shadow DOM, it is also not possible to theme the settings areas.

Questions?
Check the GitHub repo or open an issue there. Enjoy!