I saw this was an issue on a older post Tab bar colour too black
I am having the same issue, but I found the culprit. I was playing with my settings in my Mac and found Accessibility>Display>Increase Contrast. After I clicked it and then unchecked it I noticed the darker Tab bar on Brave. I went to another user on my Mac and the bar was normal color until I did the same thing. Now no matter if I check or uncheck the “Increase Contrast” button the Tab Bar is way too dark to see my tabs. Would that be a Brave Bug or MacOS bug?