The code snippet you showed is HTML and therefor invalid within the CSS based Story Stylesheet.
...wanted to center the content
eg. The whole story, the text within the Passage area, the background image, etc...
...and centre it relative to which element?
note: The HTML page of the SugarCube 2 documentation shows the main HTML elements that make up the default layout of the page. The HTML elements that make up the contents of the Passage being shown are inserted within the div element with an class of passage and one method you could use to centre all the text within that area is to add CSS like the following to your Story Stylesheet area.