Out of the box, Jekyll doesn't have very good support for the LESS CSS preprocessor. There is a plugin version linked from the offical Jekyll site, but it doesn't work as expected. I suspect that's because the plugin doesn't actually write any content when Jekyll expects it to, so the generated CSS files are removed after being written.
So, I took the original plugin, and modified it a little to write the CSS content when it is expected to.
Here's the full plugin