If you use both the Arc GTK theme and Mozilla Thunderbird as your e-mail app, we’ve found a theme that helps them blend.
The Arc theme for Thunderbird faithfully translates the soft edges and gentle colour palette of the standard Arc GTK theme to Mozilla’s open-source email client.
We’ve seen Arc themes for Firefox, VLC, Telegram, KDE and even Windows 10, but the latest app to be given some Arc love is Thunderbird, the popular open-source desktop e-mail client.
The Arc Thunderbird theme is currently in an ‘alpha’ pre-release so don’t expect a fully fitted finish. That said, if you use the Arc theme for Firefox, this Thunderbird theme will appear near identical.
You can download the latest release of the Arc Thunderbird theme as an .xpi file from the project’s Github page.
To install the theme head to the Add-ons section in Thunderbird, click the ‘cog’ icon and proceed to install the add-on manually via the Install Add-on from file… dialog. You will need to restart Thunderbird to apply the theme.
H/T Sergio