For some reason, this started as a simple task that wasn't that simple: build Bootstrap from its LESS sources using Rhino. No Ant, Maven, Play, etc.
This, I found, is the easiest and fastest way if you already have Java installed.
First, clone this Git repo: https://github.com/asual/lesscss-engine
Then build the project with mvn install and finally build the LESS files:
java -jar /tmp/lesscss-engine/target/lesscss-engine-1.4.0-SNAPSHOT-cli.jar bootstrap.less bootstrap.css
On a side note, modifying Bootstrap's LESS files with custom changes (i.e. fonts) is a very bad idea.