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

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>