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For information regarding the application itself or customization of the content within the application, we highly recommend checking forums and user guides made available by the project behind the application.
In this case, as stated in the PrestaShop 1.7 release note:
version 1.7 is a major step forward for PrestaShop, with a number of major features and forward-looking changes: the introduction of the Symfony2 framework, a rebuilt Theme system which brings a new default theme along with a Starter Theme, a brand new Product Creation page, a reworked Modules page, a reorganized main menu, … and all this while having a look at performances! This is what we mean when we say “major version”
So it is normal that previous themes or modules don't work in this new major version.