Not sure if this would prove useful today, a little older:
Apparently a qt5 theme? Couldn't be sure about that though..
Did you install Dolphin from Software? I see two variants, one ZorinOS package, one Flathub; the Zorin package has multiple options and extensions at the bottom:
Gotcha - I would think that the qt5ct application would be able to handle it. But, don't use Dolphin so I could also be wrong. That's just going off what I was able to find on it - still looking a little further but does seem like a configurable item.
Well there are plenty of themes for dolphin on plingstore: Pling Store Click the Download button and open the app.
Download your preferred Dolphin theme in the app and click the file icon at the top. Then find the theme and click apply!