I forgot to mention that I had to modify the file
/packages/layout-managed-subsite/lib/configure-subsite-integration.tcl
35c35
< 1 {subsite::set_theme -theme default_plain}
---
> 1 {subsite::set_theme -theme layout_managed_subsite_plain}
/packages/layout-managed-subsite/lib/configure-subsite-integration.tcl
35c35
< 1 {subsite::set_theme -theme default_plain}
---
> 1 {subsite::set_theme -theme layout_managed_subsite_plain}
Cheers,
Claudio