It may happen that theme paths are not loaded by the design engine.
This is due to the fact that TwigThemePass compiler pass checking the existence of twig.loader.filesystem service. Problem is that very service being private, it can be inlined by Symfony at compile time (if it's referenced only once), making it unavailable from the container at all.
The issue occurred with Symfony v2.8.25 (probably occurs with Symfony 3.x as well). Themes for templates won't be parsed at all since the compiler pass won't execute (hence the blocker priority).
The fix consists on checking ezdesign.twig_theme_loader instead, which decorates twig.loader.filesystem.
See https://github.com/ezsystems/ezplatform-design-engine/pull/10 (already merged). A version 1.0.1 will be very much appreciated