The theme of dashboards can be set to either Dark or Light. This setting can be set or edited in the Admin CSP by clicking Dashboard Builder > Dashboards >select 'Edit' next to the dashboard.
In the Explore Page Marquee section you can select the check box for Dark theme or deselect the check box for Light theme. Dark theme is meant for darker images and light theme is meant for lighter images.
Example of Dark Theme:
Example of Light Theme: