We use the pelican assets plugin, which uses the webassets python module:
pip install webassets
You'll also need to modify your pelicanconf.py to include the plugin, something like this should do the job:
PLUGIN_PATH = '/path/to/pelican/plugins/directory'
PLUGINS = ['assets']
We use bower to install bootstrap, font-awesome and the like.
Install node.js and npm using either your operating systems package manager or download it.
npm install bower
bower install
We use sass to generate our CSS files.
Install ruby using either your operating systems package manager or download it.
bundle install