With 1.5.1 update, tab tree came up this morning on both of my systems with a dark theme for the tab pane, even though that wasn't my selection earlier. (Firefox 54.0, Ubuntu 16.04.) I checked tab tree preferences, and yes, it showed Dark, but that wasn't my choice previously. I changed the setting, and all was fine. So it seems there's a minor bug here that when tab tree upgrades to 1.5.1, it changes the theme setting.
With 1.5.1 update, tab tree came up this morning on both of my systems with a dark theme for the tab pane, even though that wasn't my selection earlier. (Firefox 54.0, Ubuntu 16.04.) I checked tab tree preferences, and yes, it showed Dark, but that wasn't my choice previously. I changed the setting, and all was fine. So it seems there's a minor bug here that when tab tree upgrades to 1.5.1, it changes the theme setting.