I tried,
app/design/frontend/Magestore/sample/composer.json
{
"name": "magento/sample-module-theme",
"description": "N/A",
"require": {
"php": "~5.5.0|~5.6.0|~7.0.0",
"magento/theme-frontend-luma": "~100.0",
"magento/framework": "~100.0"
},
"type": "magento2-theme",
"version": "1.0.0",
"license": [
"OSL-3.0",
"AFL-3.0"
],
"autoload": {
"files": [ "registration.php" ]
}
}
app/design/frontend/Magestore/sample/registration.php
\Magento\Framework\Component\ComponentRegistrar::register(
\Magento\Framework\Component\ComponentRegistrar::THEME,
'frontend/Magestore/sample',
__DIR__
);
app/design/frontend/Magestore/sample/theme.xml
<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
<title>Magestore Sample Theme12</title>
<parent>Magento/luma</parent>
</theme>
app/design/frontend/Magestore/sample/Magento_Customer/layout/customer_account_login.xml
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
layout="1column" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceBlock name="customer_form_login">
<action method="setTemplate">
<argument name="template" xsi:type="string">Magestore_Sample::form/login.phtml</argument>
</action>
</referenceBlock>
</body>
</page>
app/design/frontend/Magestore/sample/Magento_Customer/templates/form/login.phtml
<?php
echo $welcomeMessage = "Login Page";
?>