If you are browsing different through the Style menu and click on a new style, it will be applied to your site. You can revert to your previous style by clicking on the Revert Template button. This will revert your style to the one you last used on your site and (if applicable) your previous custom banners and the background will be applied.
Please note: This is only an option if you have not yet logged out. When you log out of the Sitebuilder, the style that you have selected is saved as your default.
If this happens, and you want to return to a previous style, go to Style > Change Template and scroll through the styles until you locate the one you previously used, then click on it to apply it to your site. When you do this you will not lose any content from your page. However, if you have added a custom background or banner, you will need to re-add these if the new style allows for this option.