The Extended Collection properties will show you what characteristics will be used for each collection and when all or multiple collections are selected in the LUNA Viewer. 

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Site level characteristics 

Site level characteristics are identified in the column called Collection were the values are (-1) and (All Collections) . This is where you can edit the Collection Description, Theme (default, blue or white),  Header Graphic and the Default Group you see on the Luna Viewer when you have more than one collection selected or are looking at all collections.

Under the more tab, you will fine Extended Collection Properties

Click on one of the two entries you will need to edit (-1) and (All Collections) then choose the Edit Selected button.

Here you can Change the Collection Description, Header Graphic URL Default group to load in the Featured Media and the theme.

For the Header Graphic, copy the URL from the Collection Manager and paste it here.

For the theme type in one of the following default, blue or white

To make these changes take affect you will need to you will need to do a Soft Reset under the more tab. If you don't have the Soft Reset option. Talk to your LUNA administrator and ask them to do it.