SeaOwl for Thunderbird

A single userChrome.css that brings the SeaOwl palette to Mozilla Thunderbird. Every surface tracks the same OKLCH tokens as the design system — cool dark navy backgrounds, brand cyan on selected folders and the current calendar day, muted slate for sender names and header labels. Unread threads stay loud; read rows fade. Install once, survives upgrades, no third-party add-on required.

1. Open Thunderbird, then Help → Troubleshooting Information → click Open Profile Folder.
2. Inside the profile folder, create a chrome/ subdirectory if it does not already exist.
3. Download userChrome.css below and drop it into that chrome/ directory.
4. Tools → Settings → General → scroll to Config Editor, then set toolkit.legacyUserProfileCustomizations.stylesheets to true.
5. Restart Thunderbird. The cyan accents should appear on the folder pane, message list, and calendar.