For DIY merchants, this is the one area of Magento where flexibility is key. In order to get up and running quickly, merchants will likely need to rely on the built-in theme (with minimal color/style/graphic modifications), install a community theme via Magento Connect or purchase a premium theme. Free and paid themes are available from a number of developers/vendors.
In addition to themes available on Magento Connect, there are also many premium themes available which typically include support (sometimes subscription based) and a host of popular Magento extensions already installed (i.e. packaged with the theme).
Below are some resources for popular ready-made designs/templates: