BirdOne
One-line layout for Thunderbird: tabs and unified toolbar in a single row, a fixed amber accent instead of the Windows colours & one calm surface grey. Tested on Thunderbird 152+.
One line for everything
BirdOne merges the Thunderbird tab bar and unified toolbar into a single row: tabs on the left, toolbar buttons on the right, next to the window controls. The chrome shrinks to one line, and your mail gets the vertical space back. Below 850px window width the layout falls back to the default two-row interface, so a narrow window stays usable.
Colour as signal
A fixed amber accent replaces the Windows accent colour: selection highlights, the primary button and focus rings stay amber regardless of the OS setting. In dark mode, menus, toolbar icons, column headers and compose controls answer hover with amber text instead of a background block, and the message list selection uses a subtle tint with a thin amber outline.
Pure userChrome.css
The whole theme is one userChrome.css stylesheet plus an optional userContent.css for the mail body and compose editor – no extension, no background process, nothing phoning home. Folder pane, message list and reading pane share a single surface grey, and every knob is a CSS variable at the top of the file.