You can re-edit styles you applied in Microthemer via the Selectors menu in the top left of the interface. The General folder will contain any selectors you created previously, if you didn’t specifically organise them into a different folder.
Is that what you were looking for? Or am I on the wrong track?
You can just clear the values. With dropdown menus, you can select the first empty option to clear a field’s value, this can be quicker than selecting the text and using the delete button on your keyboard.
Once you’ve deleted a value in the Microthemer interface, the original style value set by your theme will apply again. Microthemer doesn’t delete the original styles, it overrides them.