In the theme options there’s an option to hide the text altogether.
I don’t *think* you will be able to place it somewhere else just using CSS as it is normally hard-coded into the theme. You would be able (obviously!) to put it in the header image before you upload it though.
The option to remove is only available in certain themes unfortunatly. Where you uploaded the custom header there’ll be a window with the text, and buttons to his the title and sub title. If they arent there the only other option is to go to your options page and delete the blog title and subtitle and have it go nekkid