Hmm.. Okay, so:

* Adding my congelado to allowed_themes in config/local.config.php did not change anything
* Adding the theme to the allowed_themes in the database produces no entry on the admin panel, too, but it switches it on. So the db entry is the value of the admin panel setting. Now I can choose the theme in my user settings.

If I do so, no difference is noticed on the UI. I added a slightly modified head.tpl to that theme, but I see the old header of the frio theme.

But when I log out, the page switches to complete white, nothing is displayed.
I need to revert my theme choice back to frio in my users db record.

So.. This does not seem to work at all. Not with the theme.php I quoted above at least.
Did anybody tried to inherit a theme for Friendica recently with success?