Need help: Basic-captcha not working in Skeleton theme

Hi all,
I need some hint tweaking one of the Skeleton themes, namely https://github.com/getgrav/grav-theme-ceevee

I have the site running nicely, but I don't get the Contact form of that Theme working with base-captcha.
I tried to understand the way the theme builds its form, but it all looks somewhat different from the modular page descriptions in the Plugin's dokumentation.

Does anybody with a clue have a minute to look into this? I'm as confident as afraid it's easy in the end, but I don't get it at the moment.
I tried to overwrite the form.md file with a simple example form from the documentation and have it displayed, but when I ad the base-captcha block nothing changes. I neither see a captcha query, nor is it (negatively) validated in the background.

Thanks for any help!

L.B.Q.R.

Start with the Grav Ceevee theme's Contact form and its form.md, then compare the base-captcha example in the form plugin documentation with the theme's form structure. Confirm that the captcha query appears and that invalid input is rejected when the form is submitted.

Written by the indexing model from the issue text.