Thank you for your reply, Richard. So far I never created child themes and I don't know how much time it takes to set up a child theme. When I briefly looked at the Moodle docs for child themes I was impressed by the length of the document and realized that it can't be done fast.
It's not a pressing need at our site. Only one student asked whether she can customize colors and that was just for fun. Although I don't need the feature I wish it would be possible to offer some color settings in the user profile or as a separate page for all users if they selected a boost theme or a boost child theme.