If you want create a custom theme
please follow below steps
Create a theme directory
To create the directory for your
theme:
- Go to <your Magento
     install dir>/app/design/frontend.
 
- Create a new directory named according to your vendor
     name like: /app/design/frontend/myvendor.
 
- Under the myvender directory, create a directory named according to your
     theme. (e.g. mytheme) theme directory structure looks like below
 
app/design/frontend/
├──
myvender/ 
│   │  
├──mytheme/
│   │  
│   ├── etc
│   │  
│   ├── media
│   │  
│   ├── web
|   |  
|   |    ├── css
|   |  
|   |    ├── images
|   |  
|   |    ├── js
│   │  
│   ├── registration.php
│   │  
│   ├── theme.xml
*etc/media/web
folder copy to install magento directory 
vendor/magento/theme-frontend-blank
Declare your theme
First need create mytheme/theme.xml to your theme directory
app/design/frontend//
Configure it using the following
example:
<!--
/**
 * Copyright © 2015 Magento. All rights
reserved.
 * See COPYING.txt for license details.
 */
-->
<theme
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">    <title>My Custom Theme</title>
   
<parent>Magento/blank</parent>
   
<media>
        <preview_image>media/mytheampreview.jpg</preview_image>
   
</media>
</theme>
Register your theme
Your theme directory add a mytheam/registration.php
file with the following content:
<?php
/**
*
Copyright © 2015 Magento. All rights reserved.
*
See COPYING.txt for license details.
*/
\Magento\Framework\Component\ComponentRegistrar::register(
   
\Magento\Framework\Component\ComponentRegistrar::THEME,
   
'frontend/myvender/mytheme',
    __DIR__
);
Configure Your theme
Default theme.js file
<your Magento install
dir>dev\tools\grunt\configs\themes.js
/**
 * Copyright © 2015 Magento. All rights
reserved.
 * See COPYING.txt for license details.
 */
'use strict';
/**
 * Define Themes
 *
 * area: area, one of (frontend|adminhtml|doc),
 * name: theme name in format
Vendor/theme-name,
 * locale: locale,
 * files: [
 * 'css/styles-m',
 * 'css/styles-l'
 * ],
 * dsl: dynamic stylesheet language (less|sass)
 *
 */
module.exports = {
   
blank: {
        area: 'frontend',
        name: 'Magento/blank',
        locale: 'en_US',
        files: [
            'css/styles-m',
            'css/styles-l',
            'css/email',
            'css/email-inline'
        ],
        dsl: 'less'
   
},
   
luma: {
        area: 'frontend',
        name: 'Magento/luma',
        locale: 'en_US',
        files: [
            'css/styles-m',
            'css/styles-l'
        ],
        dsl: 'less'
   
},
   
backend: {
        area: 'adminhtml',
        name: 'Magento/backend',
        locale: 'en_US',
        files: [
            'css/styles-old',
            'css/styles'
        ],
        dsl: 'less'
   
}
};
After add mytheme code.
/**
 * Copyright © 2015 Magento. All rights
reserved.
 * See COPYING.txt for license details.
 */
'use strict';
/**
 * Define Themes
 *
 * area: area, one of (frontend|adminhtml|doc),
 * name: theme name in format
Vendor/theme-name,
 * locale: locale,
 * files: [
 * 'css/styles-m',
 * 'css/styles-l'
 * ],
 * dsl: dynamic stylesheet language (less|sass)
 *
 */
module.exports = {
   
blank: {
        area: 'frontend',
        name: 'Magento/blank',
        locale: 'en_US',
        files: [
            'css/styles-m',
            'css/styles-l',
            'css/email',
            'css/email-inline'
        ],
        dsl: 'less'
   
},
   
luma: {
        area: 'frontend',
        name: 'Magento/luma',
        locale: 'en_US',
        files: [
            'css/styles-m',
            'css/styles-l'
        ],
        dsl: 'less'
   
},
   
mytheme: {
        area: 'frontend',
        name: 'myvender/mytheme',
        locale: 'en_US',
        files: [
            'css/styles-m',
            'css/styles-l'
        ],
        dsl: 'less'
   
},
   
backend: {
        area: 'adminhtml',
        name: 'Magento/backend',
        locale: 'en_US',
        files: [
            'css/styles-old',
            'css/styles'
        ],
        dsl: 'less'
   
}
};
**flush your
cache
After add this file. We can see the theme in backend admin side. Content
>> Theme
Now theme is created and displayed at admin side store
>> Configuration >> General >> Design to change the theme.
Hope this will help full.