Problem/Motivation
Spotted while working on 
            
              
              
              ๐
              Remove Bartik from Drupal core
                Fixed
              
             we have some code comments that I'm sure are out of date:
\Drupal\Core\Asset\AssetResolverInterface says:
   * Themes may replace module-defined CSS files by adding a stylesheet with the
   * same filename. For example, themes/bartik/system-menus.css would replace
   * modules/system/system-menus.css. This allows themes to override complete
   * CSS files, rather than specific selectors, when necessary.
I'm pretty sure this is no longer true - instead, modules and themes must declare libraries and you can override a library in a theme.
\Drupal\Core\Extension\InfoParserInterface says:
   * Information stored in a theme .info.yml file:
   * - screenshot: Path to screenshot relative to the theme's .info.yml file.
   * - engine: Theme engine; typically twig.
   * - base theme: Name of a base theme, if applicable.
   * - regions: Listed regions.
   * - features: Features available.
   * - stylesheets: Theme stylesheets.
   * - scripts: Theme scripts.
I don't think stylesheets or scripts work any more, again replaced by libraries, and there are definitely some things missing from this list too.
\Drupal\Core\Extension\ThemeHandlerInterface says:
   *   An associative array of the currently installed themes. The keys are the
   *   themes' machine names and the values are Extension objects having the
   *   following properties:
...
   *   - stylesheets: A two dimensional array, using the first key for the
   *     media attribute (e.g. 'all'), the second for the name of the file
   *     (e.g. style.css). The value is a complete filepath (e.g.
   *     themes/bartik/style.css). Not set if no stylesheets are defined in the
   *     .info.yml file.
   *   - scripts: An associative array of JavaScripts, using the filename as key
   *     and the complete filepath as value. Not set if no scripts are defined
   *     in the .info.yml file.
Same as above.
Steps to reproduce
Proposed resolution
Remaining tasks
User interface changes
API changes
Data model changes
Release notes snippet