In fact there are certain limitations when using LESS stylesheets - e.g. system cannot create LESS stylesheets as physical files (for source control), they do no support versioning, etc., so development is not as convenient as with standard CSS stylesheets.
More information about how you can work with LESS in Kentico can be found in documentation - Module: LESS Stylesheets 1.0.
Anyway, if you wanted to use LESS preprocessor, it is available for a Kentico version 7 as well. You can download it on Marketplace - LESS Stylesheets.