Hi Alex,
Unfortunately the Editor was not designed to have different themes, so there is no easy way to make changes to the appearance of the Editor, nor is there any documentation for doing so. All of the settings for the Editor’s appearance are in the source code. If you want to experiment with making changes yourself, most of the settings should be in the files located in the Source\Editor\EditorStyle folder.
I’m not sure what your second question was referring to. I believe you can make that change in the same place, though.