Have you added the registration.php file too? That is also the required file to be added in new custom theme. If you are facing product view related issues then you can also put the file etc/view.xml from luma to your custom theme. Custom theme requires two files be added always:

1. Registration.php

    `<?php 
      \Magento\Framework\Component\ComponentRegistrar::register(
        \Magento\Framework\Component\ComponentRegistrar::THEME,
        'frontend/{{vendor}}/{{theme}}',
        __DIR__
      );`

2. Theme.xml

    `<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
    <title>Your Theme</title>
    <parent>Magento/luma</parent>
    <media>
        <preview_image>media/preview.jpg</preview_image>
    </media>
</theme>`

3. etc/view.xml (optional) : if you set blank theme as parent then you need to add this, however, I would suggest to add this file too in case you are facing issues to display product data with luma as parent. You can simply copy it from luma to your custom theme.