Theme CSS won't load
TIP: Conflict when Module with same name as Theme!

Just in case someone else runs into this, if you have a module with the same name as your theme (xxx, for example), the xxx.libraries.yml file in your theme folder is ignored. Instead, Drupal looks at the xxx.libraries.yml file in your modules folder. Any files referred to by it (css, js, etc.) are also loaded from the modules folder and its subfolders, not from the themes folder.