The top white bar is part of Windows - changing theme /appearance will change that part.
The toolbar/menu bar (gray one) is part of FF, but some of the overall appearance is still handled by the Win OS. So I would try some themes and see if it shows nicer for you..

And it could be something with Linux/windows variations.. how they display...

In Linux I can change themes/colors and it can make toolbars & the whole window environment appear very different.

Maybe someone else with Win 10 & FF will chime in..if theirs looks same as yours or not.