Magento2: How to install custom theme in magento 2

magento2
 |_ app
   |_ design
      |_ frontend
        |_ Custom
            |_theme
              |_Magento_Theme
                |_templates
                  |_root.phtml     - Copy of Luma
              |_registration.php
              |_theme.xml

Your path for registration.php is app\design\frontend\Custom\theme\registration.php

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

app\design\frontend\Custom\theme\theme.xml

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

After all setup is completed, don't forget to run deploy command for the theme,

php bin/magento setup:static-content:deploy   

Check inside your admin panel, Content -> Design -> Configuration for set your custom theme.

Remove cache and check in the frontend.