Themes to Import to a Google Site
The side menu while editing your site contains three broad areas: Insert, Pages and Themes. Besides the 6 basic themes created by Google, there is a section of this panel for Custom themes. From here you can:
Create a theme
Import theme
Edit theme
Duplicate theme
Remove theme
Rename theme
Tips on importing a Google Sites theme
If you create or save a custom theme you can then import that very same theme into a new site. This imports colors, fonts, images, settings for navigation, components, spacing and more. Importing a theme will automatically change your entire site potentially. However, it won't change custom text you've input. It is only changing the defaults, like default headings, subheadings, default backgrounds, etc.
Try a different default background color. Often this can interact with the image header to produce different results.
Change the theme fonts by editing the theme and selecting default typefaces for Titles and Headings, or Body Text.
Import your own logo in the Images section to make it your own.