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

<?xml version="1.0" encoding="UTF-8"?>
<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
    <!-- The title for your theme in human-readable format -->
    <title>My theme</title>
    <!-- The parent theme your theme will inherit -->
    <!-- Almost always Magento/blank for new themes -->
    <parent>Magento/blank</parent>
    <!-- The media node is optional and can be removed if no preview image -->
    <media>
        <!-- The preview_image node defines your theme's preview image -->
        <!-- Magento looks for a path relative to your theme's directory -->
        <preview_image>media/preview.jpg</preview_image>
    </media>
</theme>

app/design/frontend/Foo/mytheme/media/preview.jpg

<!--
This is where your preview image goes.

The core Magento themes use a JPG image that is 800x800 resolution.
-->

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

<?php
// Import the ComponentRegistrar class
use Magento\Framework\Component\ComponentRegistrar;

// Register the theme, which is considered a component, with Magento
ComponentRegistrar::register(
    ComponentRegistrar::THEME, // The type of component to register
      'frontend/Foo/mytheme', // The path to your theme relative to app/design
    __DIR__ // Look at the current working directory for the component
);