The documentation in simplesamlphp for making themes is pretty non-existent, and though it says that it supports Twig now, I can not get it to work. I have in my theme (modules/mymodule/themes/fancytheme/default/):
includes/footer.php
includes/header.php
base.twig
_footer.twig
_header.twig
index.twig
From the documentation (https://simplesamlphp.org/docs/development/simplesamlphp-theming) it explains about the new Twig templates, but there is nothing about how to move to Twig templates, just to copy the twig files across.
The theme I created is working, as if I change the php files in includes, I can see the changes come through. I'm just not sure how to use the Twig files.