What about the Compiler cache in Template Manager -> Advanced -> Clear Compiler cache
The only drawback with LESS is that you can't edit inside the Template Manager with a LESS file as it doesn't see the folder. A custom CSS would have worked just as well.
Also NEVER alter the original LESS files. Better to make a template-custom.less file and make changes there.
Please see these links to better understand how to create your own custom CSS file for our newer LESS templates:
Your first CSS-Less custom file or CSS custom file