User Style Customizations for Thunderbird Supernova

Tweak the header pane subject and sender font to be a little smaller.

Usage

To enable:

Settings > General > scroll to bottom & select Config Editor

Search for legacy

Change toolkit.legacyUserProfileCustomizations.stylesheets to 'true'

To find your profile folder launch the Thunderbird binary with the -P switch or look for it in the Thunderbird configuration folder.

On Linux it is under $HOME/.thunderbird/

Create 'chrome' directory in the profile folder and place userChrome.css in there.

Restart Thunderbird for the changes to take effect.