I fixed the theme editor to display the allowWildcard property correctly. It's default value is true, so you do not need to do anything for the wildcard import to work.
But there is another important thing, when a theme is not found then it won't apply the "-defaults" unless you specify a fallback theme like this:
<!-- the fallback theme is used when the requested theme can't be found -->
<theme name="*" ref="-defaults">
<!-- optional stuff, like setting a red background for easier debugging etc -->
That way when a theme is not found it will use the fallback theme and through it gets the wildcard import from "-defaults" which gives it's child widgets a chance to get a theme.