I created a sub-theme of the Drupal8 W3CSS theme and followed their directions about copying and renaming files, and editing the info.yml file. I installed it, and set it as default theme. It works. (I can see my custom logo.) The styles.css file is not coming from my files, but from the default sub-theme, which I haven't enabled. When I view the source of a page, I see the following tag, which doesn't contain the path for the CSS file used in my sub-theme.
<link rel="stylesheet" media="all" href="/themes/d8w3css/drupal8_w3css_subtheme/css/styles.css?qbo25b" />
It could be I missed a step.
Do I have to do something with Composer after changing any .yml file of the theme? I did clear all the cache bins.