Child themes are good idea, we thought about it before. Child theme is good if theme has its own functions that can be used in child theme. In our case if you for example create child theme and edit ad.php file. Then on next update we change ad.php file, your child theme will not reflect update because it will use its own ad.php file. So using child theme is not that much usable in this case. Only way it can be useful when you change ad.php and we update other file, then you do not have to change anything and your child theme will be up to date.
It is good idea but we are not planning to add it any time soon. It is good to have but not must have feature for us.
Currently if you want to make changes to theme you can:
copy and rename existing theme folder located in user-content/themes/. Make sure to use unique name that cannot be overwritten with other new theme, for example add prefix that is unique to you.