Do I need to use a callback function here, or is there another way?
I would suggest to take a different approach. It is complicated to get a callback for the complete loading of a CSS stylesheet loaded dynamically: see this Stack Overflow question for reference:

Is there anyway to listen to the onload event for a  element?

Do not use JavaScript to set the content position: it is part of styling and should be done in CSS instead. Isn't the role of your CSS skins to modify the appearance of the page?