Maybe I am making it to difficult. If you want you can change the bg-dark to the color of your choice. It will effect the entire theme though not just the menu. Anywhere the bg-dark color is being used it will be reset to the color you selected. Example bg-dark { background: #ff0000; }” would change everthing to red.
If you want to create a new style rule you will need to do it in the style.css file. You would simply define your color as a class .bg-<<mycolor>> where <<mycolor>> would be the color you want. example .bg-red{background-color:#f00;} if you prefer you can use predefined colors in html you can find a list here https://www.w3schools.com/html/html_colornames.asp.
So you can define your color as .bg-red{background-color:red;}.
The code where the class is defined is located in the header.php file at line 62. You will should see the line <div class=”navigation section no-padding bg-dark”> you can just change the bg-dark to the class name you defined. Example <div class=”navigation section no-padding bg-red”>
You should be doing this in a child theme not the theme itself.
