Can you be more precise? Which messages do you want to highlight? Are you using the "Table" view or "Cards" view for your thread pane? In Table view Thunderbird by default highlights the background of a '''selected''' message: • when the thread pane is in focus the highlight (background) color is blue most of the times, it depends on the accent color you choose in your OS • when the thread pane is out of focus the the background color becomes a light gray (my screen shots have been made in table view)