You must create and register a new theme:

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>

You must create and register a new theme:

You just need to create 2 files theme.xml and registration.php these are the examples for the theme in the image (Dayds). You should change that name for your own.

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>

Then go to the backend and activate your new theme for the desired store.