Firefox themes have no line to separtate browser tabs - why?

I have just moved from Chrome to Firefox, and it seems none of the 10+ themes I have tried have a way to distinguish each of the open tabs on top of the browser. It will only highlight the current tab, but not show a colour change or outline or anything to mark the boundary between one tab and another. Pity, it makes it more clear.
Is this a “framework” limitation with Firefox, or just the way people are not prioritising a visual separation of all tabs?