xspless brings support for using LESS CSS in Domino. Specifically, the current form this support takes is an Eclipse Builder for Domino Designer.
By installing this plugin, right-clicking on an application, and choosing “Add/Remove LESS CSS Nature”, it allows Designer to automatically compile LESS files to normal CSS versions. It matches two types: stylesheet resources with names in the form “foo.less.css” (because Designer automatically appends the “.css”), in which case it creates a file named “foo.css” next to it, and file resources elsewhere (e.g. File Resources or WebContent) with names in the form “foo.less”, in which case it creates a new file next to it named “foo.less.css” (to avoid overwriting similarly-named normal-CSS files).