Magento 2 - Enable theme from codebase.

project-config.php

<?php
// Project's config.php file
return [
    'system'=> [
        'default' =>[
            'design' => [
                'theme' => [
                    'theme_id' => 'frontend/vendor/theme-name', // theme name, as declared in registration.php
                ]
            ]
        ]
    ],
]

theme-registration.php

<?php
// theme's registration.php file

use Magento\Framework\Component\ComponentRegistrar;

ComponentRegistrar::register(ComponentRegistrar::THEME, 'frontend/vendor/theme-name', __DIR__);

⚠️ doesn't quite work when there is a theme set via admin panel