Magento – How to override layout file, which is already override by other theme – layout\override\base\somel_ayout.xml

When creating any new theme or module you need to define registration.php file at root of your module or theme folder.

Always use theme name in lowercase, Because Magento used this standard for theme name declaration.

You haven't any problem for keep Theme name in camelcase but use standard way is much appreciated.

You have to define registration.php file inside Magento_Theme folder, its in wrong place.

Correct diagram of theme structure will be below,

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

registration.php file :

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

your theme.xml file path will be,

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

theme.xml file :

<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.

To override layout from parent theme, here in your case default_head_blocks.xml from Magento/blank :

app/design/frontend/{Vendor}/{themeName}/Magento_Theme/layout/override/theme/Magento/blank/default_head_blocks.xml

More information in Magento DevDocs.