I've a less file inside app/design/frontend//web/css/source/_test.less and included in default.xml like
<head>
<css src="css/source/_test.css" />
</head>
It's working perfectly in developer mode but in production mode it's showing error like "
(index):1 Refused to apply style from ....
If you want to add your custom CSS you just have to declare it in that your theme layout.
XML path:
app/design/frontend/YourTheme/default/Magento_Theme/layout/default_head_blocks.xml
<?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" layout="1column">
<head>
<css src="css/custom.css" />
</head>
</page>
Less path:
app/design/frontend/YourTheme/default/web/css/custom.less
