In the 'Themes' section of your account, you will be able to add new themes and customize existing ones to create the desired look for your website.
Adding a new theme:
To view the available themes and add one to your account, click 'New' at the top of the page while in the 'Themes' section.
From there, click the theme thumbnail to preview it, and then click 'Use this theme' to add it to your account. Keep in mind that clicking 'Use this theme' will add the theme to your list of theme themes, but will not apply it to your site (it will not replace your existing theme). To apply a different theme to your site, select the one you want to add and click 'Set as Default' in the upper right hand corner of the page.
Customizing a theme:
To customize a theme, simply select the theme you'd like to make changes to from your list of themes and then click 'Edit Theme' in the top right hand corner of the page. From there, you will be taken into the theme editor where you can customize the theme. To read more about the theme editor, please click HERE.