Does Hyvä Checkout Require The Hyvä Theme?

In short, yes - you need a Hyvä Theme in order to use the Hyvä Checkout. This is due to the fact that the theme is a dependency of the checkout as the latter makes use of the features and functionality provided by the former. That being said, the rest of your store pages DO NOT have to implement the theme in order for you to use the checkout, you can use the theme fallback mechanism to configure the checkout to use the Hyvä Default theme.

To set up a Hyvä theme fallback for Hyvä Checkout, in the Magento admin panel, configure the following settings:

1. HYVA THEMES->Fallback theme->General Settings->Enable
   The configuration path is hyva_theme_fallback/general/enable

2. HYVA THEMES->Fallback theme->General Settings->Theme full path
   The configuration path is hyva_theme_fallback/general/theme_full_path
   To use the default Hyvä theme set it to frontend/Hyva/default.

3. HYVA THEMES->Fallback theme->General Settings->The list of URL's parts
   The configuration path is hyva_theme_fallback/general/list_part_of_url
   To use the Hyvä checkout, configure the path hyva_checkout/index.