The main color is Preferences > Appearance > Colors > General > Focus Zone Background, and Preferences > Appearance > Colors > Project > Project Work Area Color.
It’s probably not 100% right but here’s the screenshot. I prefer it a lot.
I tried to get the overall DAW colours right and I also tried to match the default track colours. I also imported the Logic Project colours palette file I found here on the forum.
IMHO, Cubase should provide a set of skins, at least two, a light version and a dark version.
There are so many values to a full set of Cubase Colors; working out a well-designed set is beyond me. I’ve seen some advanced users demonstrating how the color sets can be extended and perhaps one day I’ll attempt that. That said, a good set of skins would be a welcome, if cosmetic, addition.