serve.json

{
  "$schema": "https://dev.office.com/json-schemas/core-build/serve.schema.json",
  "port": 4321,
  "https": true,
  "serveConfigurations": {
    "default": {
      "pageUrl": "https://contoso.sharepoint.com/SitePages/Home.aspx",
      "customActions": {
        "fcea9230-7f22-45b7-815c-081a49474611": {
          "location": "ClientSideExtension.ApplicationCustomizer",
          "properties": {
            "cssurl": "/Style%20Library/custom.css"
          }
        }
      }
    },
    "injectCss": {
      "pageUrl": "https://contoso.sharepoint.com/SitePages/Home.aspx",
      "customActions": {
        "fcea9230-7f22-45b7-815c-081a49474611": {
          "location": "ClientSideExtension.ApplicationCustomizer",
          "properties": {
            "cssurl": "/Style%20Library/custom.css"
          }
        }
      }
    }
  }
}