Fixed header in Foster template

I've made the header fixed using below code but the header is not on the top of the page. When I scroll down I can see a little bit of the content showing through small gap at the top of the page (screenshot attached). How can I move the header to the top of the page please?

.Header {
  position: fixed!important;
  z-index: 1000;
  width: 100%;
}

https://rhombus-paddlefish-39ss.squarespace.com/

Cheers

.Header {
	position: fixed! important;
	z-index: 1000;
	width: 100%;
	top: 0;
}

also, your site is private.

I had a similar issue and the code you provided @tuanphan, it fixed the gap issue. The problem I'm having now is that the header is covering content that I previously placed on certain pages outside of the home/index page. Is there a way to push the content below the header? Whenever I try to add an element to the top, I can technically place it there but it remains covered by the fixed header.

See attached screenshots