Thank you for writing in, the configurations of the standard X menu are under Theme Options > Header there you'll see the Navbar Font, Navbar Font Size, color options, etc.
The navbar backgrounds options are under Theme Options > Renew
Yes, having a child theme is always a good idea. You might not need it for now since you're only doing a CSS edits, but as your site is getting bigger most likely you're going to add widget or something that will require a child theme.
How To Setup Child Themes
Not on themes directory but on your site database since those are page/post content/data, the images will go to the upload folder though.
You can put all your custom CSS in the Theme Options > CSS area.
Hope this shed some lights,