Sorry wrote this out and forgot to post it.
I tested out the LESS stylesheet in both the base theme and the customize plugin and it worked for me first go. I used the below code to test
If you're finding that your changes aren't showing even in the custom code area, then that would lead me to think it's the actual code that's incorrect, as the custom code box will read both LESS and standard CSS.
I've also heard that some code editors save the LESS stylesheet with a hidden .css extension so look out for that when you've saved the file. Must be Style.less and not style.less.css