Your template design is the layer that sits behind every page of your site. This is your main site design, and will likely include your site logo, social icons and menu. To make changes to the main design, you need to access the Page Master.
Click 'Design' in the main toolbar.
From the left sidebar, click 'Page master'. From here you can add content in the normal way via the 'Insert' tab in the main toolbar, including a site logo and menu. NB: Remember that anything you add here will appear on every page by default, so make sure to keep space for your individual page content.
For steps on changing the wallpaper, site size and other template settings, click here.