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Arc Theme integration for Thunderbird (alpha v0.5.0)

alt tag

Tested in Thunderbird 52.5.0, GNOME 3.26.2, Ubuntu 17.10.

Stability in other versions is not guaranteed, but you can still try.

Because this project is still in alpha state, feedback is appreciated. Due to the lack of knowledge of how things work in Thunderbird, mess-ups may be very likely, therefore everyone is welcome to report any visual bug that might be found by opening an issue on GitHub.

You can also reach me on the #arc-thunderbird-integration IRC channel on


  • Changes the appearance of the tabs and the toolbox to match the look found in the official Arc Firefox Theme;
  • changes the appearance of the Lightning add-on to be in syntony with the overall look;
  • supports Light, Dark and Darker theme variants;
  • every specific part of the integration can be configured and/or disabled through the extension preferences panel.


Version 0.5.0 comes with some fixes and new styling in several parts of Thunderbird. The extension is still in alpha state (sorry!), meaning that it may not be very stable and may contain several unforseen bugs.

Similarly to Arc Firefox Theme, this integration is meant to be used along with the official Arc Theme, you can however also use it with other operating systems and themes. If the integration is used with other OS theming, it is possible that some parts will look broken. It is possible to tweak or disable certain styling through the preferences panel.

The integration supports three variants.

From up to down: Light, Dark and Darker theme variants


  • Download the latest release (.xpi file) from here;
  • In Thunderbird, go to menu -> Add-ons;
  • There should be a button with a gear icon on the left side of the "Search all add-ons" bar, click it;
  • Click "Install Add-on from file...";
  • Find and select the downloaded .xpi file and install it;
  • enjoy.


The content in this repository is licensed under GPLv3.


  • 0.5.0 2017/12/30
    • Fixed buggy preferences panel (for real, I hope)
    • Included more styling
  • 0.4.3 2017/07/03
    • Fixed some buggy behavior in the preferences panel
  • 0.4.2 2017/06/13
    • Fixed visual bug when hovering on the close button of a tab
    • Fixed coloring of search icons for search boxes in the mails view
  • 0.4.1 2017/06/12
    • Fixed message body visibility bug
  • 0.4.0 2017/06/12
    • Added support for Dark and Darker theme variants
    • Included panel for configuring the integration in the preferences window
    • Optimized existing icons, and included new ones
    • Converted the add-on from overlayed XUL to XPCOM bootstrapped, removing the necessity to restart the application when the add-on is installed or removed
    • Applied styling in the Address Book window
    • Applied styling in the dialog used for creating or editing events or tasks in the Lightning extension
    • Applied styling in the calendar in the Lightning extension
    • Fixed compatibility issues in some external add-ons
    • Ported the styling source from CSS to LESS
  • 0.3.0 2017/04/02
    • Integrated several (but not all) icons
    • Dynamically restyled some stock icons in Thunderbird to match the general look and feel
    • Applied further style changes to the tab browser to match the official theme
    • Restyled Lightning task pane
    • Restyled buttons, fields and items in the toolbox
    • Applied some style tweaks in the mail page
    • Applied fancy borders and shadows in drop down menus in the toolbox
  • 0.2.0 2017/03/07
    • Integration packaged as an add-on
  • 0.1.0 2017/03/05
    • Initial release


This integration contains work originating from:

The icons were copied and altered to be used in the integration.

The color palette and overall look and feel comes from the Arc Theme.