Although changing the default theme is not recommended, you may, of course, do whatever they wish with your forum. The reason that changing the default theme is not recommended, is that some of your custom themes might be using files from the default theme's directory. If it doesn't find them, or finds them in a condition that it's not expecting, you might find that none of your themes will work. Or, if they work, they might not work correctly.

So, if you decide to ignore the recommendation and modify the default theme, please, make sure you take a complete backup, of it, first, just in case.

Don't worry. You don;t HAVE to actually display the default theme, if you don't want to. You can set things so that the default theme cannot be used by your members, if so-required.