I tried the customizer with no results. I went to posts-categories and looked in the address bar for the ID# and slug. I could not find the body tag and its class that you mention above. So, I put the <style> info that you have above into the head of my header template and got the color I want for each category. Does this sound right or did I just stumble into the wrong path that worked anyway? Also, the whole page doesn’t fill with the color, can I make the total screen area my color?
Thanks again and please note that this is a theme that I am creating from WordPress For Dummies, by Lisa Sabin Wilson 8th edition.