The newest features:

• Add a site logo to the global header
• Set background colour for logo and navigation
• Pick a theme colour based on site logo
• Specify a custom theme colour

These features have been released to personal Google Sites and G Suite domains that use the Rapid Release and takes 1-3 days for roll out to complete. For G Suite domains that use Scheduled Release you can expect to begin to see the feature on 02 May 2017.

Add site logo and choose background colour

When you hover your mouse over the page header you get an Add logo option where you can upload or select a logo for your site that will appear in the navigation area of the site see Site Logo.

Custom theme colour

Once you upload a logo you can Edit logo to pick a colour based on the logo. Alternatively you can pick any accent colour for the site from the Theme panel and then use the custom colour picker: Custom Theme Colour.