I created a new theme. I want to use the Luma theme as the base theme.
theme.xml
<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
<title>My Template</title> <!-- your theme's name -->
<parent>Magento/luma</parent> <!-- the parent theme, in case your theme inherits from an existing theme -->
</theme>
But it looks like this: