Is it just a background change? If so, you can do this already with CSS on MediaWiki:Common.css or MediaWiki:Monobook.css
Could you please upload a screenshot to show how it looks like (I don't have a personal MW installation to test myself)?
(Almost) everything in CSS can be changed via Common.css/<youskin>.css.
I can help you if you want, but I'm on Discord.
However, if you want a faster solution, you can inspect the element via your browser's element inspector. Right click somewhere on the page > Inspect Element (or just F12) > find the element you want to change > apply a style there (if there's a class, ID or role) > if satisfied, apply the changes to Common.css (assuming you already know some basic HTML and CSS)
Your modified version of "MonoBook" is unfortunately declined. It would cause fatal errors with the existing MonoBook skin because the skin name is the exact same (in skin.json and the classes in /includes/*) . It would cause class name and skin name conflicts. In order to have a modified skin of MonoBook, it'll require more extensive modifications to remove conflicts. Your best option will be CSS at the moment to achieve this.