Theme developers require sufficient documentation to create new themes for Drupal 8 or porting their Drupal 7 themes to Drupal 8.
This documentation will form part of https://www.drupal.org/theme-guide/8
This documentation should cover:
- .info.yml file structure - https://www.drupal.org/node/2349827
- directory structure of themes - https://www.drupal.org/node/2349803
- basics of Twig (or link to good documentation of this) - https://www.drupal.org/theme-guide/8/twig
- how to get JS included in the pages of a site - https://www.drupal.org/node/2216195
- Approaches to working with CSS - https://www.drupal.org/theme-guide/8/adding-stylesheets
- how to update a theme from Drupal 7 to 8 - https://www.drupal.org/update/theme
- template overriding (suggestions) - https://www.drupal.org/node/2354645 / https://www.drupal.org/node/2186401
- differences with Drupal 7 - https://www.drupal.org/node/2356951
(Theming experts: Anything else that's essential?)
Still to Be Completed:
While much of the documentation now exists some is either 'Incomplete' or 'Requires Technical Review'.
- Defining a theme with an .info.yml file https://www.drupal.org/node/2349827 Incomplete
- Template naming conventions https://www.drupal.org/node/2354645 Needs Technical Review
- Working With Twig Templates https://www.drupal.org/node/2186401 Needs Technical Review
- Theming differences between Drupal 6, 7 & 8 https://www.drupal.org/node/2356951 Incomplete
- Twig in Drupal 8 https://www.drupal.org/theme-guide/8/twig All the 'Twig in Drupal 8' sub-pages Require Work
Once these documents are marked as 'No Known Problems' this issue should be marked as fixed.