Issue #1987066 by Kevin Morse, JohnAlbin, rteijeiro, echoz, ry5n: Rename files to match CSS file naming convention.

Issue #1806478 by Cottser, joelpittet, kevinquillen: Make twig the default engine once all modules templates are converted from .tpl.php to .html.twig.

Issue #1975462 by thedavidmeister, alexpott, chx, joelpittet: Fixed Allow and test for NULL and integer 0 values in Twig templates.

Issue #1033116 by eojthebrave, c4rl, brianV, tim.plunkett | webchick: Rename template.php to THEMENAME.theme to elimnate ambiguity around 'the template file' vs. 'template files'.

Issue #575298 by sun, tim.plunkett, Rob Loach, KrisBulman, Jacine, danillonunes: Provide non-PHP way to reliably override CSS.

Issue #1696786 by Fabianx, jenlampton, stevector, steveoliver, jwilson3, amateescu, chx: Integrate Twig into core: Implementation issue.

Issue #304540 by Rob Loach, marcingy, typhonius, BrockBoland, jaredsmith: Fixed Disable themes when theme engine or base theme arn't available.

Issue #1538478 by Niklas Fiekas: Register lib/ directories of themes as PSR-0 roots for Drupal$theme namespace.