Pop-up styling differences between web map and story map

[AndreaConver]

I've created a custom pop-up using html in my web map. The pop-up appears as expected in my web map, but it looks slightly different in my story map. 

Web map on the left and story map on the right:

Can anyone tell me why there's a difference between the two environments? Also, any tips for getting the blue header bar with the (1 of 2) to display in my story map?

Thanks!

Andrea