You can find out all of those things yourself. The font is listed in
Settings -> Editor. The background color is in the source of your syntax theme. You can also open the developer tools at
View -> Developer -> Toggle Developer Tools. All of the CSS rules that determine how Atom is displayed are visible there, and you can use ctrl-f to search for
<atom-text-editor>, which will have those style rules attached to it.
You appear to be asking about fonts, but in English, when you ask for the “source” for a computer program, people are going to think that you’re looking for this.