I notice that when you mouse over a tab it goes distinctively grey but once activated it turns to indistinguishable black. Why not keep it as grey? That would surely be an easy fix. Contrast this with the dark theme in Vivaldi for example. The latter is fine.
I tried it on Ubuntu (18.04.2 LTS) and it works as desired. So it may well be a macos specific issue.
In fact, I believe that, in macos, the app code may need to specifically direct change-window-focus clicks to the app if it wishes the app to act on the click beyond the change of window focus.
Obviously possible though, given Chrome, Firefox and Safari all do it.
I am running macos 10.14.4 (Mojave).