Thunderbird

Thunderbird honors Firefox-style profile CSS: chrome/userContent.css styles content documents (message body, calendar event description editor), chrome/userChrome.css styles the application UI.

Dark theme: calendar editor visibility

With the dark theme, the calendar event description editor can render dark text on a dark background. Override it in userContent.css.

Enable user styles first: Settings → General → Config Editor, then set toolkit.legacyUserProfileCustomizations.stylesheets to true. Restart Thunderbird after creating or editing the file.

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Cheatsheet

Open the profile folder

Help → Troubleshooting Information → Profile Folder → Open Folder. Create chrome/ inside it if it doesn’t exist.

chrome/userContent.css

body {
  color: #e0e0e0 !important;
  background-color: #2b2b2b !important;
}
 
a:link {
  color: #8ab4f8 !important;
}
 
a:visited {
  color: #b39ddb !important;
}