Shortlist template Header Background

[NourddineMaghmouli]

Hello,

I have downloaded the compiled version of the shortlist template(storymap-shortlist/README.md at master · Esri/storymap-shortlist · GitHub ), I hosted it in my C drive(IIS), I managed to change the header background image but it does not fit under the Tabs, I guess it needs some transparency but I am not very familiar with changing the html and css code, can someone please help me how to achieve that? 

Thanks

Noureddine