We're looking into ways to let themers define their own Twig filters and functions, and one way to do it would be to allow themers to define their own Twig Extensions within their themes. This ought to be possible using a services.yml file, together with a patch such as the one at, but it looks like these files are only supported by modules right now.
Is there any indication that services.yml files might be supported by themes?
FAILED: [[SimpleTest]]: [MySQL] 56,485 pass(es), 5 fail(s), and 0 exception(s).
FAILED: [[SimpleTest]]: [MySQL] 56,473 pass(es), 7 fail(s), and 2 exception(s).
FAILED: [[SimpleTest]]: [MySQL] Drupal installation failed.