Selecting a theme style preset
To select the theme style you would like to use in your store, click over to the Theme settings tab, then scroll all the way to the bottom to the Change theme style button. Here, you can preview the theme styles and select the one that's right for your store.
Applying and saving a new theme style will overwrite your current style settings, however will not remove content from your theme. It's always a good idea to make a backup copy of your theme before you start making customizations. Use the duplicate copy as your working version until you're ready to publish it.