Bootstrap left LESS and went to SASS when you upgrade from version 3.0 to version 4.0. If you just know LESS, it’s not a big deal, really, to learn SASS. They’re both pretty similar and end up producing the same results: cleaner and easier to scale and use CSS.
There are a lot of tutorials about LESS and SASS, you shouldn’t be too concerned. If you’re not a command-line guy, there are utilities and software to save your .less, .sass and .scss files into .css in the same place you expect them to be. Also, it’s not recommended using the server listener utility to pre-compile your CSS from these source files on the production server because it’s processes that, like Symfony’s Assetic where assets get compiled and packaged in dev, belong only in the dev environment. So by the time you are ready to push your LESS and SASS live, it’s simply part of your code deployment to the live site, not a big deal.
Here are some excellent links and tutorials that helped me understand LESS, SASS and their main differences. In the end, they’re just pseudo languages designed to create more consistent and clean CSS.
Video about LESS, pre-compiled CSS
Here are some useful links and downloads for learning more about LESS
http://winless.org/ – this is the free software to create LESS in Windows
Koala: A useful editor for LESS, SASS and more
I suggest also trying Koala: A free, multi-purpose CSS pre-compiler editor
Other useful LESS and SASS resources
Using Compass and SASS together
Last comments about SASS and LESS
It’s sometimes overwhelming, just trying to keep up with the fast-paced changes and new emerging tools and libraries out there. SASS and LESS are quite similar and produce the same results in the end. I am favoring SASS because of how it is trending and also how the design is a little less cumbersome.
By “design” I’m referring to the language syntax and structure, it’s a little easier to create more with less, but not with LESS, SASS.
Okay, I was trying to be a little “cutesy” above, but it is true. SASS is winning the CSS pre-compiler competition and competition is exactly what this is, which is a great thing for the web, web developers and all users of the Internet because competition like this is what will make the Internet better. From here, more tools, methods and languages will likely emerge in the future, perhaps just replacing CSS with something more programmatic, flexible and efficient. Keeping your design directives separate from your code is a good thing, but these design directives really need to be modernized.