In Sakai 2.9 it seemed much more clear current site you were on as the button was darker than the background and stood out. In Sakai 10, the border is just slightly darker.
This was one of the biggest remarks in early testing that this active site button should be more apparent. I'm not sure what the best look for OOTB should be? A different color or thickness to the border? A different color background/text? I think this is a quick fix but just not for me.