If you don't care about the source and just want a .dll you can get a compiled release from https://github.com/dotless/dotless/downloads
Simply select for the latest successful build and click on the "Artifacts" section, here you'll find the latest compiler exe and any dll's required.
This is a project to port the hugely useful Less libary to the .NET world. It give variables, nested rules and operators to CSS.
For more information about the original Less project see http://lesscss.org/. For more information about how to get started with the .NET version see http://www.dotlesscss.org/.