Use below code forFirst you need to check theme is applied or not. You can check your layout customer_account_logincurrent theme by ctr + U, you can see current theme path in .css and .js file.xml
In your case Magestore/Sample display in .css and .js file path. If display it, theme is set properly.
<?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">Magento_Customer::form/login.phtml</argument>
</action>
</referenceBlock>
</body>
</page>
Run below command
php bin/magento setup:upgrade php bin/magento setup:static-content:deploy php bin/magento cache:clean
Now you can see your changes.
Let me know if you have any query.