I have also problem understanding how this is supposed to work.
How should a widget that supports light/dark theme be updated? If using the colorScheme then it will not be in sync with the home screen's light/dark. The home screen's light/dark theme seems not accessible to the widget.
It seems a bit odd if widgets are supposed to follow the system's theme while the icons follows the home screen's theme. Furthermore, I know that at some point it worked in a more logical manner, but seems to have changed close to release of iOS 18.