From the document https://npp-user-manual.org/docs/plugin-communication/#nppn-darkmodechanged it is stated that hwndFrom in message NPPN_DARKMODECHANGED is hwndNpp. However, it is 0 instead. The problem is that in NppBigSwitch.cpp, when this notification is sent it uses lParam as hwndFrom, like this:

		case NPPM_INTERNAL_REFRESHDARKMODE:
		{
			refreshDarkMode(static_cast<bool>(wParam));
			// Notify plugins that Dark Mode changed
			SCNotification scnN;
			scnN.nmhdr.code = NPPN_DARKMODECHANGED;
			scnN.nmhdr.hwndFrom = reinterpret_cast<void*>(lParam);
			scnN.nmhdr.idFrom = 0;
			_pluginsManager.notify(&scnN);
			return TRUE;
		}

And in the code, most places send NPPM_INTERNAL_REFRESHDARKMODE with 0 as lParam, e.g. Notepad_plus::refreshDarkMode().

The code in NppBigSwitch should be changed to use hwnd as hwndFrom.