Magento 2 Luma Theme – How to Add Block to Header

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.

Customizing Header in extended LUMA Theme

1. Adding a <DIV />` around your logo and giving styles:

   Edit logo.phtml and put a Div around the logo. Assign a class to the DIV and define styles to this class in your CSS files.

   app/design/frontend/vendorname/themename/Magento_Theme/templates/html/header/logo.phtml
   

   Use _extend.less for adding custom styles

   app/design/frontend/vendorname/themename/Magento_Theme/web/css/source/_extend.less
   

2. Edit your search box in your extended module, extend if not extended. Add a DIV around it and assign our class and define styles in _extend.less

   app/design/frontend/vendorname/themename/Magento_Search/templates/form.mini.phtml
   

3. The same method as above you can use for Navigation at the below path in sections.phtml

   app/design/frontend/vendorname/themename/Magento_Theme/templates/html/header/sections.phtml
   

I hope this will help you

If you need to move element in top bar then check my answer on this question How to move correctly block-search and minicart to panel wrapper? Magento 2

Run upgrade/deploy/cache command to see the CSS changes if you are using _extend.less for custom styling