Change Background Color for “Next Post”

The main color for my site is black. These seems to be the color setting for the background color of “next article” or “previous article” at the bottom of each post.

https://mentalhealthmiracle.com/testosterone-and-mental-health/

I want my header to be black, but don’t want this black background over the “next” and “previous” parts. Is there any way to change this?

How come this site has the main color set as grey, yet they don’t have the same grey background over the next and previous posts?

https://roamtheworld.online/recipe/make-homemade-taho/

Maybe I screwed something somewhere by changing something in style.css, but I don’t think so because I reverted to the original fashionista theme (I’m using a child theme) and I still had the same problem.

Thanks!