We have one project which has a custom theme in vendor folder. Like vendor/{Vendor}/{custom-theme}/

vendor/{vendorname}/custom-theme/registration.php

<?php

\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::THEME,
    'frontend/Vendor/customtheme',
    __DIR__
);

Now I need to override some files from the custom theme. So I created theme folder in app/code/design as per registation.php

Override `vendor/{vendorname}/{custom-theme}/Magento_Catalog/templates/product/list.phtml

to

app/code/design/Vendor/customtheme/Magento_Catalog/templates/product/list.phtml

But unfortunately, not working.

I know the one way to override vendor custom theme files, create and register new theme with a parent vendor custom theme to make this work.

But I would like to know if there is any other way to work this without creating another theme?

Note: the Custom theme is developed by previous company and they do not provide composer credentials to update custom theme from the composer. So we must override custom vendor theme to app/code/design