Google Sites provides many different themes for you to choose from, as well as a way for you to further customize them.
Click on More > Manage Site
On the left sidebar, click on Themes
There are many different themes available that you can choose from.
Click on Preview to view how the themes would look like on your website.
Click on a theme you would like to use and then Save Changes.
To customize your site further, click on Colors and Fonts.
Here you can control specific elements of your site, like adding your background image, changing your font, the colors of your text links, etc. You can see all of your changes on the Preview box before saving.
Once you are done customizing, click on Save Changes.
