Themes control the public design and presentation of a website.
Choosing a theme: If you want to change the design theme from the default choice, choose from the theme options in the Appearance tab [The Appearance tab is located at the top right of your screen when you are on your dashboard]. With the free account option, you have 7 themes to choose from. To switch to a different theme, click on the green Use This Theme button under the thumbnail of the theme you've chosen.
Configuring a theme: Once you have settled on a theme, click the “Configure” button to upload logo, add headers and footers to the pages, add text to the site’s homepage, or reorder and rename the site’s main navigation. Configurations are unique for each theme and will be saved with the theme. You must configure each theme when changing designs. But if you return to the original theme, all of your initial configurations will be saved.