I need to load the css file THEME\Magento_Checkout\web\css\checkout\index\index.css in the checkout page only.
So I created the file THEME\Magento_Checkout\layout\checkout_index_index.xml and added a layout instruction:
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="1column" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<head>
<css src="Magento_Checkout::css/checkout/index/index.css"/>
</head>
</page>
I can see in the source code that it tries to load the script, but if I open it then I get
Compilation from source: LESS file is empty: frontend/company/base/de_DE/css/checkout/index/index.less