Hello @KathrynHailes , the color of that line between the header and story is not something that is customizable. If you don't want the line to appear at all, your only option at the minute would be to remove the header completely, which you can accomplish by appending ?header=false to your published story.
If the customization of the line between the header and story is something you want to see in the story design settings, I'd recommend posting on the ArcGIS StoryMaps Ideas page to request the enhancement from the StoryMaps team.
--Abby