By default, an Open edX installation has one site for users to interact with. You can configure multiple sites within your Open edX installation. A site presents Open edX courses and content in an individual way. If you set up multiple sites, you can configure them independently of each other. For example, you can assign a different theme to each site and specify which courses are available on each site.
You host each site on a separate domain or subdomain from your Open edX installation. You configure the domain name for a site in the Django admin site when you create it. For example, you might configure one site with the domain name university.edu and another site with the domain name advancedplacement.edu. Or you might configure one site with the domain name arts.myuniversity.edu and another site with the domain name sciences.myuniversity.edu.
create_site configure_site
For more information about themes, see Theming Open edX.