The site library can be customized, however, it is highly recommend that tweaks and changes be handled by someone with CSS and HTML knowledge, as well as someone with a keen sense of design.
The site can be designed, within limits, from the banner to the site footer. Logos, links, and general content can be designed to match your website, and integrated seamlessly within your existing branding guidelines.
Clicking the "Design" link in the Site Dashboard's main navigational menu will direct you to customization tools for your site header, the banner and text associated with it, along with the content, and footer tweak areas.
Without getting into too much technical detail (you should seriously have some CSS skills), you can change design, logos, favicons, links, and layouts from within this section of the Guide.