For the very first time, SilkUI isn't loading properly. I don't know what I'm doing different (as far as I know, nothing - and I have in this server other projects using it just fine).
Problem solved! Instead of creating an empty layout, just clone one that it's used on the themes and adapt. I was missing key "includes"!
You are absolutely right. The problem was that you were using a custom layout without some of Silk UI requirements.
You can create your own layout, but you need to have the WidgetsForLayout pattern on the layout, and also you need to have a container enclosing everything with a specific set of classes: