Hyvä is never active directly "as a theme" on a page - it always works through two layers: a parent theme (Hyvä's actual core package) and your own child theme, where your project's customizations live. That's the same inheritance logic Magento themes generally use - the parent theme just happens to come from Hyvä instead of Luma.

Which parent theme variant?

Hyvä offers two variants of the default theme: hyva-themes/magento2-default-theme (without built-in CSP support) and hyva-themes/magento2-default-theme-csp. For projects that want to enforce a Content Security Policy - generally a good idea - the CSP variant is the right choice. In this tutorial, and in the mironsoft project itself, that's the parent theme:

hyva-themes/magento2-default-theme-csp

Installing via Composer

In the mironsoft project, every Composer command runs through the bin/composer wrapper instead of directly - that way the command is guaranteed to run inside the Docker container with the right PHP version. First, the Hyvä module and the CSP theme itself:

bin/composer require hyva-themes/magento2-theme-module
bin/composer require hyva-themes/magento2-default-theme-csp

magento2-theme-module brings Hyvä's own Magento modules (including the CSP integration itself), magento2-default-theme-csp is the actual parent theme with all the default templates.

Creating your own child theme

Next we create the child theme where all project-specific customizations live - in this project it lives under src/app/design/frontend/Mironsoft/default/.

app/design/frontend/Mironsoft/default/
├── registration.php
├── theme.xml
├── composer.json
└── web/
    └── tailwind/
        ├── tailwind.config.js
        ├── package.json
        └── src/
            └── styles.css

<?php

declare(strict_types=1);

use Magento\Framework\Component\ComponentRegistrar;

ComponentRegistrar::register(
    ComponentRegistrar::THEME,
    'frontend/Mironsoft/default',
    __DIR__
);

<?xml version="1.0" encoding="UTF-8"?>
<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
    <title>Mironsoft Default</title>
    <parent>hyva-themes/default-csp</parent>
    <media>
        <preview_image>media/preview.jpg</preview_image>
    </media>
</theme>

The <parent> entry is the crucial line: it tells Magento that our theme inherits all templates, Layout XML, and base configuration from hyva-themes/default-csp, as long as we don't explicitly override anything (chapter 10 covers the override conventions in detail).

Registering and activating the theme

bin/magento cache:clean
bin/magento setup:upgrade
bin/magento config:set design/theme/theme_id 2  # ID of the new theme

You can find the actual theme ID either in the admin UI under Content > Design > Themes, or via the CLI. Alternatively, you can select the theme directly in the admin under Content > Design > Configuration for the desired store.