Can I use Hyvä Commerce without Hyvä Theme?

Technically yes, the Admin Dashboard and Media Optimisation modules don’t depend on the Hyvä Theme storefront. You can install Hyvä Commerce on a Magento store running the default Luma theme. In practice, almost every Hyvä Commerce store also runs Hyvä Theme, because once you’ve adopted Hyvä’s tooling on the admin side you usually also benefit from the storefront performance gains.