I don't think it's very easy to edit the styling of the editor for newbies, so I thought about releasing the dark version of my editor I made or let's say edited.
1. Download the rar I attached below.
2. Unrar the downloaded file.
3. You'll see a folder called "jscripts" in it. Just move it to the root directory of your MyBB installation and overwrite the files.
4. Make sure you're using the correct editor. AdminCP > Templates & Style > Themes > Your Theme > Scroll a bit down and you'll see "Editor Style" - Choose Monocons.