In this post I answer the question: “I would like to change the background color of my site header”.
I would first check which theme you are using.

My quick way: check the html source (usually right click => view page source). Searching for “wp-content/themes/” would give me an instant clue.
I would also cross check with one of the online Theme checkers online. E.g. https://www.wpthemedetector.com/ reveals wpbuffs.com is using Elementor.

How to proceed next, depends on the configuration of the website.

My preferred way: “The Theme way”
If the theme builder provides a way to do change the background color of the side header, I would recommend that first.

Other way: “The CSS way”
If they already are using a Child Theme, I would suggest to add the CSS over there. If not if the Theme provides a way to add custom css.

Make a Child Theme using one of the Child Theme plugins and custom the CSS.
Though having child theme purely for styling is not my preferred solution because that way: styling configurations might be too much “all over the place”.