To change your theme navigate to Website > Design > Theme. Select the theme you would like by clicking the radio button next to the corresponding theme.
Theme options are only available for Venture and Boundless themes. They allow you greater customization of header/footer content, as well as the menu colors scheme for your website. Venture also gives the ability to set the side-menu content that displays next to the banner. Once you've selected your options, be sure to hit Save!