The Themes section allows you amend the look and feel of the pages. There are a number of different pre-set themes which range from simple colour and font changes, to full overhauls of how a page looks.
These are the following presets which are available in the Themes section. When choosing a specific Theme, it will update the whole Google Site rather than just one page. It would be best practice to go through each page one you have changed the Theme to see if this is how you want the site to look.
Each preset has a different option to change an accenting colour and allows you to change the colour scheme.