Need help: Basic-captcha not working in Skeleton theme

[lebochequirit]

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.