What's the "Header Separator Color" in Confluence for theme customization?

[Ruth Norton]

I'm trying to make our JIRA and Confluence look the same, but in JIRA there is a setting to:

"Header Separator Color"

Is there an equivalent for Confluence?  Right now I have a white background but there needs to be a grey border underneath, otherwise the header looks weird with the body but I can only modify the border on JIRA and can't find it on Confluence.

Additionally, when I inspect, I see that it's specifying a border-bottom color, but no setting where I can modify this.