Non-default SharePoint themes are not restored with the site application. The original default theme is applied to the site collection and then you must manually apply a non-default theme.
Backup Exec SharePoint Agent only restores the default theme to the site collection.
Use SharePoint Central Administration to create a site collection. This applies a default look and feel theme to the site collection, and then change the theme of the site collection.
Refer to the Microsoft SharePoint documentation for more information about applying themes to a site.