Using Less in the plone.app.theming editor only works at root

If we copy and modify a Diazo theme throught-the-web using the Theming editor, and if this theme contains some .less files, we can declare them in `portal_less` with an id like:
`++theme++my-ttw-theme/style.less`
and it works fine.
But if the .less file is not at the root, like:
`++theme++my-ttw-theme/less/style.less`
Then it does not compile.

I haven't find a fix for now, but I know this is due to https://github.com/plone/Products.ResourceRegistries/blob/master/Products/ResourceRegistries/tools/BaseRegistry.py#L672 which produces an Unauthorized error.