There should be Makefile targets added for compiling CSS files for the Bootstrap theme from LESS files. The general idea is that during installation, the compilation would occur automatically. There should also be a script available, which would presumably be invoked by the Makefile targets, so that users of Git-mode installations can recompile at any time. This is meant as a tool for installation and distribution of changes -- it is not necessarily expected that users (yet) would be editing the LESS files as part of customizations; instead, relying on *UserCSS and external stylesheets would still be the norm. This may change.
Link to some notes that Owen wrote up: http://wiki.koha-community.org/wiki/Working_with_Bootstrap_OPAC_LESS_files
One suggestion by Owen is using Grunt <http://gruntjs.com/> as part of the toolchain.
My worry is the "let's add this technology" may raise the bar required for development and testing. A nice link on how to use the technology would be required for those willing to test but not knowledgeable in these arcane mystical things.
*** This bug has been marked as a duplicate of bug 19474 ***