First of all, don't blame CSS and that "no one else" excuse for trying "nothing in particular" - meaning nothing.
The code you finally included shows a location with bg-dark so one could guess that's what you are replacing but maybe not. And how are you doing the replacement? With your keyboard? With DOM ?
For that matter, do you know what, if anything, is part of the bg-dark class? You really still tell us next to nothing.
Your comment about CSS is such that either you've little to no experience with CSS or you hope someone will do the work for you. Here's the work.