So I'm not sure if this is actually a bug or simply an oversight, but I figured it... probably fits here?
It's pretty much what it says on the tin. In the image below, I'm on the screen to select a new member for a hunting party. I have Den 3 selected, but it's impossible to tell. If I simply hover my mouse over the tab, it gets darker; if I actually click on the tab, as I did in the image below, the tab remains the same color as the others. This is especially confusing if I'm not paying attention when I click the tab, as I have no idea which tab I'm looking at.
The same is not the case for light mode, which clearly indicates which den tab is currently being viewed by making it slightly darker than the others (in this case, I've selected the same tab as the above image, Den 3). Hovering over tabs without clicking on them on light mode makes them darker as well, as it does with dark mode.