I have this structure :
|-app/design/frontend
|-Magento
|-Theme
|-layout
|--default.xml
|--default_head_blocks.xml
|-Magento_Theme
|-templates
|-web
|-css
|--custom-m.css
|-js
|-theme.xml
|-requirejs-config.js
|-registration.php
In default.xml :
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceBlock name="head">
<block class="Magento\Theme\Block\Html\Head\Script" name="requirejs" before="-">
<arguments>
<!-- RequireJs library enabled -->
<argument name="file" xsi:type="string">requirejs/require.js</argument>
</arguments>
</block>
<!-- Special block with necessary config is added on the page -->
<block class="Magento\RequireJs\Block\Html\Head\Config" name="requirejs-config" after="requirejs"/>
</referenceBlock>
<referenceBlock name="logo">
<arguments>
<argument name="logo_file" xsi:type="string">images/logo.png</argument>
</arguments>
</referenceBlock>
<remove name="report.bugs"/>
</body>
</page>
And in default_head_blocks.xml
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<head>
<meta name="viewport" content="width=device-width, initial-scale=1"/>
<css src="mage/calendar.css"/>
<script src="requirejs/require.js"/>
<css src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" src_type="url"/>
<css src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap-theme.min.css" src_type="url"/>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js" integrity="sha384-smHYKdLADwkXOn1EmN1qk/HfnUcbVRZyYmZ4qpPea6sjB/pTJ0euyQp0Mk8ck+5T" crossorigin="anonymous" src_type="url"/>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.bundle.min.js" integrity="sha384-u/bQvRA/1bobcXlcEYpsEdFVK/vJs3+T+nXLsBYJthmdBuavHvAW6UsmqO2Gd/F9" crossorigin="anonymous" src_type="url"/>
<css src="Magento_Theme::css/custom-m.css"/>
</head>
<body>
<referenceContainer name="after.body.start">
<block class="Magento\Framework\View\Element\Template" name="head.polyfill" as="polyfill" template="Magento_Theme::js/polyfill.phtml" before="-"/>
<block class="Magento\Framework\View\Element\Js\Components" name="head.components" as="components" template="Magento_Theme::js/components.phtml" before="-"/>
</referenceContainer>
</body>
</page>
But my custom-m.css in not detected. Any ideas ?
EDIT :
In fact, the problem is from my default_head_blocks.xml. Bootstrap is not charging inside this (i added cdn in phtml file, but i forgot), so i'll try to figure out the problem