Fix Sticky Header

[miggs125]

What happened:
The frontend's sticky header seems to remove any bottom margin when fixed at the top of the page. This needs to be fixed so that the main text area does not jump and stick to the header when it is fixed at the top.

What should have happened:
The sticky header should appear seamless. Not the current behaviour

How to reproduce it (as precise as possible):
npm i
cd src/frontend
gatsby develop

go to localhost:8000 and scroll. Pay attention to the header as you scroll