Because DzlTab doesn't behave as correctly as GtkStackSwitcher with regard to the needs-attention dot. Particularly, the shadow is white for adwaita-dark but GtkStackSwitcher behaves correctly in this case and use a dark shadow. This commit removes the shadow totally as even if($variant=='light…) didn't work when testing.
It is really strange that the tabstrip isn't a GtkStackSwitcher because it would basically do the job and we would have a working dot shadow and the dot would be next to the icon (which I didn't manage to do with the current DzlTab(Strip)).
This is what we had and what we have now:
(Note that it works well with Adwaita, the problem is with other themes (i.e. Adwaita-dark) because the shadow color is hard-coded to white)