Module provides helper functions for working with stylesheets. @function Synchronously loads a style sheet from `uri` and adds it to the list of additional style sheets of the document. The sheets added takes effect immediately, and only on the document of the `window` given. @param window {nsIDOMWindow} @param uri {string, nsIURI} @param [type="author"] {string} The type of the sheet. It accepts the following values: `"agent"`, `"user"` and `"author"`. If not provided, the default value is `"author"`. @function Remove the document style sheet at `sheetURI` from the list of additional style sheets of the document. The removal takes effect immediately. @param window {nsIDOMWindow} @param uri {string, nsIURI} @param [type="author"] {string} The type of the sheet. It accepts the following values: `"agent"`, `"user"` and `"author"`. If not provided, the default value is `"author"`. @function Verifies that the `type` given is a valid stylesheet's type. The values considered valid are: `"agent"`, `"user"` and `"author"`. @param type {string} The type of the sheet. @returns {boolean} `true` if the `type` given is valid, otherwise `false`.