With the introduction of hub sites, two or more modern SharePoint Online site collections can now be brought together under the same global navigation and theme. 

To connect a site to a registered hub site:

1. On the homepage of the site to be connected, select the gear icon (top right corner) > Site information.
2. From the Hub site association dropdown, select the desired hub > Save.​
3. Once connected, refresh the site to view the inherited global navigation and theme from the hub site.

If your site isn't a hub, register it as one by contacting the SharePoint team.

Related Information

Microsoft has published the following article(s) on this topic:

• SharePoint hub sites new in Office
• Associate a SharePoint site with a hub site