I installed Zorin Core, and played with it. While on Core, I also installed Ungoogled Chromium, and its window match the dark theme I have chosen to have with Zorin, just like it does with the pre-installed Firefox.
I decided to support Zorin's development, and paid to download the Pro version. Well, Ungoogled Chromium isn't dark as it was on Core. It has the grey colour instead. Is that a known bug? Did I dreamed it being darn on Core, and was not, actually?