You must create and register a new theme:

[![enter image description here][1]][1] 


  [1]: https://i.sstatic.net/ZNf1c.png

To works you just need to create 2 files theme.xml and registration.php these are the examples for the theme in the image.

> app/design/frontend/Dayds/default/registration.php

    <?php
    \Magento\Framework\Component\ComponentRegistrar::register(
        \Magento\Framework\Component\ComponentRegistrar::THEME,
        'frontend/Dayds/default',
        __DIR__
    );

> app/design/frontend/Dayds/default/theme.xml

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