here my default.xml
<body>
<referenceContainer name="header-wrapper">
<block class="Magento\Framework\View\Element\Template" name="customeline" template="Magento_Theme::customeline.phtml"/>
</referenceContainer>
<referenceBlock name="logo">
<arguments>
<argument name="logo_file" xsi:type="string">images/mytheme-logo-snake.png</argument>
<argument name="logo_img_width" xsi:type="number">80</argument>
<argument name="logo_img_height" xsi:type="number">80</argument>
</arguments>
</referenceBlock>
<referenceBlock name="report.bugs" remove="true"/>
<referenceBlock name="catalog.compare.sidebar" remove="true"/>
<referenceBlock name="wishlist_sidebar" remove="true"/>
<referenceBlock name="copyright" remove="true"/>
</body>
here my phtml file
<div class="block" style="float: left;">
<?php echo __('this is custom theme'); ?>
</div>
/frontend/Bigstore/Theme/Magento_Theme/layout/default.xml
/frontend/BIgstore/Theme/Magento_Theme/templates/customline.phtml
error:-
exception #0 (Magento\Framework\Exception\ValidatorException): Invalid template file: 'Magento_Theme::customeline.phtml' in module: '' block's name: 'customline'