Magento 2 – How to Override Footer Template in Custom Theme

In default.xml from module Magento\Theme add this code inside footer referenceContainer

<block class="Magento\Cms\Block\Block" name="block-footer" after="footer_links">
      <arguments>
            <argument name="block_id" xsi:type="string">block_footer_custom</argument>
      </arguments>
</block>

Goto backend Content > Blocks > Add new static block with identifier name block_footer_custom with any html code you need. It so easy to editable Clear cache and re-deploy static. Now you will get your custom block

====UPDATE====
Call block inside template

<?php echo $block->getLayout()->createBlock('Magento\Cms\Block\Block')->setBlockId('block_footer_custom')->toHtml();?>

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.