Facebook container extension - information appears wrong

On the Facebook container extension page (https://addons.mozilla.org/en-GB/firefox/addon/facebook-container/) it says that in operation “The tab is coloured blue to indicate you are in the container.”

That’s not what I see, with Mac OS dark mode. The tab is light grey when selected, like all the other tabs, and dark grey when not.

The color is actually the bar below the tab title in the last screenshot, which seems black. You can choose the color of containers in about:preferences#containers