Frontend module configuration below /conf/SITENAME/sling:configs.json

sling:configs.json

{
    "jcr:primaryType": "nt:unstructured",
    "com.adobe.aem.wcm.site.manager.config.SiteConfig": {
        "jcr:primaryType": "nt:unstructured",
        "themePackageName": "SITENAME-theme"
    },
    "com.adobe.cq.wcm.core.components.config.HtmlPageItemsConfig": {
        "jcr:primaryType": "nt:unstructured",
        "prefixPath": "/content/SITENAME.theme/_default",
        "css": {
            "jcr:primaryType": "nt:unstructured",
            "element": "link",
            "location": "header",
            "attributes": {
                "jcr:primaryType": "nt:unstructured",
                "rel": "stylesheet",
                "href": "/css/theme.css",
                "type": "text/css"
            }
        },
        "javascript": {
            "jcr:primaryType": "nt:unstructured",
            "element": "script",
            "location": "header",
            "attributes": {
                "jcr:primaryType": "nt:unstructured",
                "async": "true",
                "src": "/js/theme.js",
                "type": "text/javascript"
            }
        },
        "favicon": {
            "jcr:primaryType": "nt:unstructured",
            "element": "link",
            "location": "header",
            "attributes": {
                "jcr:primaryType": "nt:unstructured",
                "rel": "icon",
                "href": "/resources/images/favicon.png",
                "type": "image/png"
            }
        }
    }
}

• Every theme deployed by the front end pipeline has a name defined in its package.json file.
• When a theme update got successfully deployed, AEM knows which site uses the deployed theme thanks to the themePackageName of the SiteConfig node.
• Then, to update the site using the newly deployed theme, AEM updates the prefixPath of the HtmlPageItemsConfig node with the uniquely timestamped path where the updated theme was deployed.