Anything that you have in the
/static/ directory is copied over by Hugo as-is. (So without any modifications of sorts.)
For your situation, that means that the
/static/css/ folder gets copied by Hugo as the
/css/ folder at the root of your website (
And so if a theme file should refer the CSS file you put in
/themes/my-theme/static/css/, you can just use the
href="/css/mystyle.css" notation to access it in a theme file.
(Other approaches would be to type the full address, use the
.Site.BaseURL variable, or use the functions for generating relative and absolute URLs.
But that are all details and ideas for later. Regardless of which specific approach you want to use and prefer, the
/themes/my-theme/static/css/mystyle.css file gets copied over by Hugo as
Hope this helps.