Some external libraries are needed for captchas to work (image processing). We use the wheezy.captcha Python
which on its turn requires PIL or Pillow libraries to work with.
On a debian-based system, the following will be adequate:
apt-get install libjpeg libjpeg-dev libfreetype6 libfreetype6-dev zlib1g-dev
Afterwards, install Pip requirements as usual:
pip install -r theme-requirements.txt