How to change header background color

The theme that I used for my blog came with a photo that took up approximately 60% of the header, and the rest was a gray background with the RSS feed symbol on it, and the blog title. What I’d like to do is change the background color from gray to black, so it will match the background color of my blog logo. Can someone please tell me how to go about doing that? I’m not sure if it will be in the “Header” section of my theme, or if it will be in the CSS style sheet. Also, I don’t know the color code for the gray that is there, or for black, so if you could tell me what the code for black is, and where to insert it, that would be greatly appreciated.

My blog can be found here: https://steelerstoday.com/