I have seen other tickets related to this, and have followed recommended steps. I have not been able to see new custom theme in admin section.

Here the steps I have done:

1. I have created folder structure under 'Magento root'/vendor/myvendor/mytheme/

[![Folders structure][1]][1]
  [1]: https://i.sstatic.net/VIooZ.jpg
.


2. Then I have added file: 'theme_dir'/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>
.

3. Then I have added file: 'theme_dir'/composer.json


>     {
>         "name": "myvendor/theme-frontend-mytheme",
>         "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"
>             ]
>         } }

.
4. Then I have added file: 'theme_dir'/registration.php :

    <?php /**  * Copyright © 2016 Magento. All rights reserved.  * See COPYING.txt for license details.  */
    
    \Magento\Framework\Component\ComponentRegistrar::register(
        \Magento\Framework\Component\ComponentRegistrar::THEME,
        'frontend/myvendor/mytheme',
        __DIR__ );
.

5. Then I updated '<Magento root>'/dev/tools/grunt/configs/themes.js - just below luma:


        luma: {
            area: 'frontend',
            name: 'Magento/luma',
            locale: 'en_US',
            files: [
                'css/styles-m',
                'css/styles-l'
            ],
            dsl: 'less'
        },
        my_theme: {
            area: 'frontend',
            name: 'myvendor/mytheme',
            locale: 'en_US',
            files: [
                'css/styles-m',
                'css/styles-l'
            ],
            dsl: 'less'
        },

.

6. Then deleted var from root

7. Then I flushed cache

8. Then I ran from CLI - in this step the new theme was not shown in logs:
    magento setup:static-content:deploy

9. Then when I go to Admin/Content/Themes and I am not able to find new theme.