Select the door and switch to "Style" in the right property panel, switch on the "Stone Threshold" option to activate the threshold stone. After that, click the material icon under the "Stone Threshold" button to replace the material.
Select the door and switch to "Style" in the right property panel, switch on the "Stone Threshold" option to activate the threshold stone. After that, click the material icon under the "Stone Threshold" button to replace the material.