I'm beginning with Magento and it's incredible that create a theme is so complicated...
I've understand that to add my css to the head I need to override a layout. So I override default_head_blocks.xml with this content :
<?xml version="1.0" ?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<head>
<!-- Add your own CSS files -->
<css src="css/styles.css" />
</head>
</page>
But this not works. Also after : - ./bin/magento setup:upgrade - ./bin/magento cache:flush && ./bin/magento cache:clean
vehicules
. I'm not sure if this is required or just a convention but try changing that.