The first iterations of the v3 framework were built with LESS, but the inefficiencies of compiling it's output soon became obvious, so LESS was scrapped in favour of Sass, specifically the .scss syntax.
The two leading pre-processors are remarkably similar in many respects, so if you are familiar with LESS will have no trouble in quickly understanding what Sass is doing. You will also quickly understand from viewing the output of Sass why it has quickly become the only choice for professional front-end developers.
One of the key aims of Oneweb has always been performance and, as Sass generates a more succinct and efficient output it was clearly the best choice. So, If you haven't yet tries Sass, you should, it's a joy to work with. Grab a copy of Oneweb and dive right in!
Don't like pre-processors?
I shan't waste my time trying to convince you, you can still use Oneweb by simply editing /css/style.css in the old-fashioned way.
You are missing out on a lot of the magic though…