Where to change CSS for small header issue

I have added a new header by changing the kubrickheader.jpg file to my own header, and set my header colour to white in options.

There is a small white margin at the top of my header that I want to dis appear. This is obviously part of the original header as when I change the header colour in options , this small strip also changes colour.

Where/what do I remove/change in the code to make this happen?

Here is my site:

https://www.steel-city.co.uk/blog/

So i want the red bar to be right at the top of the page.

Thanks in advance for comments ??

Cheers