Here you can find detailed information about each theme available for you to choose. Each theme has a different look and feel so check out which one suits your taste the best. You can only use one theme/design per site but you can switch the theme whenever you wish to.
If you have already chosen a pre-built design template, changing the theme will have a dramatic effect and will likely require significant cleanup. If you note which theme your original template used before selecting another, you can switch back to that theme to recover your template design.