This is an attempt to recreate the faithful old Plastik windeco from the days of KDE 3.5.x, which lives on as Trinity or TDE.
I will say this up front: it is impossible to replicate the look of Plastik exactly, because of how Openbox handles the Gradient and SplitGradient primitives; this has a bit of Polyester/Lipstick or even Crystal vibe to it as a result. However, it's close enough that it shouldn't stand out too much. I am also aware that there's not quite enough border around the entire window (close/min/max buttons) seem a little too far to the right; this was the best I could do.
You need to use a size 11 font for this to work, I've found. Noto Sans 11 Bold is on display in the screenshot. Using smaller fonts makes the close/min/max buttons shrink and look weird. Again, this is down to how Openbox handles its windeco buttons; there doesn't seem to be a titleheight option in an Openbox .themerc file as there is in Fluxbox or IceWM, for example.