stylelint - How to validate CSS within Custom XML file

Question

I have a set of xml files which stores the CSS syntax in <CSS> section of the file.

I want stylelint to validate that CSS within those files.

However, when I run the command npx stylelint /**/*.xml it gives no errors even if there are errors.

I tried using .stylelintrc.json

{
    "extends": ["stylelint-config-standard"],
    "overrides": [{
        "files": ["*.xml"],
        "customSyntax": "postcss-html"
    }]
}

Is there any way to validate CSS inside the custom tag of the XML file?

TIA.