Yes, almost all css are in less syntax. Why?

but do you use LESS?
when looking at the loaded resources of your demo-site it doesn't look like using LESS.

e.g. from your override.css:
** This file is generated by php and placed into theme_supermint / elements / overrides.css.php

Hi,

All less is pre processed by php and outputed in main.css
You're right, Override.css is not less pre processed no need for this little file.