To override LUMA consider as parent theme  you need to change `theme.xml` file in folllowing location :

    app\design\frontend\vendor_name\theme_name\theme.xml

using below code

    <theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
         <title>Your Theme name</title> <!-- your theme's name -->
         <parent>Magento/luma</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>