Hi everyone, just started playing with theme creation in magento 2 and want to use Luma as a starting point. Instead of copying this theme I want to create a theme based on luma, but can't seem to get it to work.

Steps:

created folder /app/design/frontend/mycompany/mytheme

created /app/design/frontend/mycompany/mytheme/theme.xml

<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
     <title>My main theme</title>
     <parent>Magento/luma</parent> 
 </theme>

created /app/design/frontend/mycompany/mytheme/composer.json

{
    "name": "mycompany/mytheme",
    "description": "My theme",
    "require": {
        "php": "~5.5.0|~5.6.0|~7.0.0",
        "magento/theme-frontend-luma": "100.0.*",
        "magento/framework": "100.0.*"
    },
    "type": "magento2-theme",
    "version": "100.0.2",
    "license": [
        "OSL-3.0",
        "AFL-3.0"
    ],
    "autoload": {
        "files": [
            "registration.php"
        ]
    }
}

created /app/design/frontend/mycompany/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/mycompany/mytheme',
    __DIR__
);

Unfortunatly this does not seem to work.

How can I create a theme based on a theme?

To explain, it loads all the files (but none of the CMS blocks) and when I create a different file web/images/logo.svg it still takes the file from Luma