In your Main.hs, do this:

main :: IO ()
main = do
    ld <- newLessDirectory'     "stylesheets"
    td <- newTemplateDirectory' "templates"   $ withSplices splices
    quickServer $ lessHandler     ld lessReloadHandler    $ \ls ->
                  templateHandler td defaultReloadHandler $ \ts ->
          ifTop     (render     ts "index")
      <|> dir "css" (renderLess ls)
      <|> fileServe "static"

Now any requests matching /css/foobar.css will get served the result of putting the file /stylesheets/foobar.less in your project directory through lessc. The output of lessc is cached, so it isn't run on every request. If you want to refresh the cached stylesheets, send a request to /admin/reload/less. If you want a different reload handler, pass your own one to lessHandler instead of lessReloadHandler (or simply (const pass) if you wish to have no reload handler).