Hi,
We still need to create our theme in app/themes in Sylius 1.3, but with the new symfony structure, app doesn't exist. (cf: https://docs.sylius.com/en/1.3/components_and_bundles/bundles/SyliusThemeBundle/your_first_theme.html)
As @pamil said on slack :
It'll be better to use the templates directory or create a top level directory themes.

Sylius version
1.3

Describe the proposed solution
Use the templates directory like templates/themes

Describe alternatives you've considered
Or create a top level directory themes