Why isn't my sub-theme using its CSS file? 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.

&lt;link rel="stylesheet" media="all" href="/themes/d8w3css/drupal8_w3css_subtheme/css/styles.css?qbo25b" /&gt;


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.
In Drupal 8 you setup your subtheme libraries in subtheme.libraries.yml. These libraries are then loaded from subtheme.info.yml.

You should check the paths in those files if they are correct. 

It's good practice to search for all occurrences of the base_theme's machine_name inside your subtheme folder and replace those with the machine_name of your subtheme.