This issue has novice tasks. If you are an experienced core developer and have multiple commit mentions, please review novices' work on these tasks rather than doing them yourself. Feedback from experienced contributors is valued.
The listing at https://api.drupal.org/api/drupal/core%21modules%21system%21theme.api.ph... includes templates defined in core themes, and it shouldn't. To be more helpful it should only list the default definition of each piece of markup/output: https://drupal.org/node/1823416#docblock
You can check Drupal 7's .tpl.php files as a comparison.
Some or all core theme templates (templates under
/core/themes only) include the line
@ingroup themeable, which should only be added to default theme output (in core this means output/templates defined by a module).
Remove the @ingroup themeable and line above from all core theme templates:
* * @ingroup themeable
- Write patch (novice)
- Review patch to check it fixes the issue, the change is properly documented and for coding standards. Provide test evidence (novice)
- Keep issue summary up to date (novice)
User interface changes