Twigs and Sticks and Themes and Bones - but names will never hurt me!
Trumpets and drums. Hear the call!
[TL:DR - read this instead: Twig Status Update and Call for Contributors and start here: Convert core theme functions and templates to Twig]
The Drupal7 Theme system is impenetrable. Drupal8 will very hopefully change all that, but we really need to rally the troops now and get it done.