I think you may be trying to draw a technical distinction which misses the point of what people mean by “client-side decorations” and “server-side decorations”.

I think you may be trying to draw a technical distinction which misses the point of what people mean by “client-side decorations” and “server-side decorations”. Win32 has always defaulted to server-side decorations, by which we mean that the window manager controls the rendering, appearance and functionality of the title bar. Maybe this is actually run in client space via user32.dll or whatever as part of the OS-managed event loop, but functionally your app gets told “here, have a client area” (and isn’t that name telling!) and works inside that and completely ignores the remainder of the window area, and the window decorations implementation is provided by the window manager, so that theme changes (ancient Classic, XP Luna, recent Classic, Aero Glass, Modern, colours within each, &c. &c.) immediately apply globally. This is what people mean by server-side decorations. They mean what is still the default under Win32 and I presume macOS, even if on both platforms it’s not the recommended style any more for most apps. But it’s also worth noting that the recommended client-side decorations style on Win32 at least (don’t know much about macOS) is still guided by window manager stylistic conventions, including theme colours where possible.