magento2 Custom Theme Sample Theme

Example

Theme.xml

app/design/frontend/Magento/mytheme/theme.xml

<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
     <title>My theme</title> <!-- your theme's name -->
     <parent>Magento/blank</parent> <!-- the parent theme, in case your theme inherits from an existing theme -->
     <media>
         <preview_image>media/preview.jpg</preview_image> <!-- the path to your theme's preview image -->
     </media>
 </theme>

app/design/frontend/Magento/mytheme/composer.json

{
    "name": "magento/theme-frontend-blank",
    "description": "N/A",
    "require": {
        "php": "~5.5.0|~5.6.0|~7.0.0",
        "magento/theme-frontend-blank": "100.0.*",
        "magento/framework": "100.0.*"
    },
    "type": "magento2-theme",
    "version": "100.0.1",
    "license": [
        "OSL-3.0",
        "AFL-3.0"
    ],
    "autoload": {
        "files": [
            "registration.php"
        ]
    }
}

app/design/frontend/Magento/mytheme/registration.php

<?php
/**
* Copyright © 2015 Magento. All rights reserved.
* See COPYING.txt for license details.
*/
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::THEME,
    'frontend/Magento/mytheme',
    __DIR__
);

at the end

php bin/magento setup:upgrade