make exhibit home page layout empty because this layout isn't for 'real' pages. Page with "DPLA Exhibit Homepage" layour is container of exhibit meta-information (thumbnail, thumbnail caption and exhibit description)
Exhibition Overview Page: use ExhibitHomepage for displaying thumbnail and thumbnail caption
Exhibition Overview Page: display themes thumbnail on exhibit overview page instead of default image
Exhibition Overview Page: display ExhibitHomepage description as exhibit description
Exhibition Overview Page: do not display pages with layout "Exhibit Homepage" on overview page