moodle/theme/bootstrap/less

If you want to make changes to the .css generated from these .less files then you
need to install recess (https://github.com/twitter/recess) to compile the .less files,
then run these commands in the bootstrap/less/ folder:

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

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

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

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

Non-command line options for .less compilation are available, see their github for more info.

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.