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"and in this moment we see what is underneath"

that's true if we are not in themed mode but in themed mode that is not enough, we have to tell theme service to Draw the background of the control's parent and that's what method DrawParentBackground do (actually by calling the API function DrawThemeParentBackground)
because in themed mode we use DrawElement procedure to Draw (actually by calling API Function DrawThemeBackground)and with this function if you want transparency you must use DrawThemeParentBackground function before.
more detail: