If you want to make changes to the .css generated from these .less files then you
need to use a LESS Compiler. Details on specific compilers and how to install them
can be found at http://docs.moodle.org/dev/LESS

At present, recess is the official tool for developers to compile LESS in to CSS, as
other tools give different results in the generated CSS.

Once recess has been installed you can compile and compress the LESS in to CSS using
the following commands from the "theme/bootstrapbase/less" directory:

For the main Moodle styles:

recess --compile --compress moodle.less > ../style/moodle.css

And for the subset of styles of interest to the TinyMCE editor:

recess --compile --compress editor.less >  ../style/editor.css

You can add --watch to make sure it updates every time you make a change.

If the compilation is failing and you're not getting any useful error message, try using lessc instead i.e.:

lessc moodle.less

This is the same tool that's getting called by recess, but the errors seems better if you go direct.

Additional information about the Moodle bootstrap base theme can be found at
http://docs.moodle.org/dev/Bootstrap