Just a quick note for my own records.
When you're having trouble getting your sub theme's styles.css file to show up, and you've read over the official docs until your eyes bleed, and you've double-checked that your .libraries.yml file and .info.yml files match what they're supposed to match, here's one more thing you can check:
Does the parent theme have any schema or settings to install? As in theme/config/schema/theme.schema.yml and theme/config/install/theme.settings.yml.
That was my issue this morning. Once I copied, and renamed "theme" to my sub-theme name, I everything started working correctly.
Really wish the official docs had mentioned this one...