Installation

1. Download

Download userChrome.css and userContent.css

2. Enable custom stylesheets

In Thunderbird go to Settings → General, scroll to the bottom and open Config Editor, then set:

toolkit.legacyUserProfileCustomizations.stylesheets = true

3. Keep the tab bar visible (recommended)

In the same Config Editor set:

mail.tabs.autoHide = false

so the tab bar (and with it the one-line layout) is always visible, even with a single tab.

4. Find your profile folder

Help → Troubleshooting Information → Profile Folder → Open Folder

5. Copy the files

Create a chrome folder inside the profile folder if it doesn’t exist, then copy these files into it:

• userChrome.css — Thunderbird UI styling
• userContent.css — mail body and compose editor in dark mode

6. Restart Thunderbird

The layout applies on restart.