I have seen this question a lot, and always with an answer to deploy content or a missing .htaccess in pub/static or to clear cache, processed files in var,
Did all of this without success.
I created a a custom them, or better copied the folder from another of my MAGENTO DIRECTORY.
Permissions and ownership seems alright, they have to be if on pub/static the Luma and Blank are creating files with no problem, I have also custom Js on my custom theme, all deployed with no problem.
If I work with Grunt (uninstalled now) all LESS files are there, all but these 3 CSS files in the subject. They should be inherited by the Luma theme by configuration anyway.