Data would not be changed by installing a new theme. Any pages or post data you have will convert over to the new look of the theme. But if you have made any CSS style changes (in the editor) those changes will be lost in the new theme. Most themes will save that information if you go back to the orignal theme.
What is the theme you downloaded. Under the appearance menu you might have editing options.
What Theme do you have? The Theme is a submenu on Appearance. Click on it and post the theme name.
That worked thank you for your help.
