File style.css is produced by the lesscss compiler - lessc. Secondly, style.css is used in production mode - with an entry ASKBOT_CSS_DEVEL = True in the settings.py file. When ASKBOT_CSS_DEVEL setting is absent or set to False, the source style.less will be loaded and compiled by the browser. A side-effect of that is a possible momentary flash of unstyled content and some delay in the page load.
ASKBOT_CSS_DEVEL = True is a convenient setting for the designer.
Please find documentation about the lesscss format elsewhere.
The following command will compile the lesscss source file, an option -x will produce compressed css file:
lessc file.lesscss -x > file.css