My questions are:
Do I need to learn how to create a whole theme?
I read this answer -- Sublime 2 -changing background color based on file type? -- for Sublime 2, but I'm using Sublime 3 (currently in beta), and there is no "Color Scheme - Default" in the Package folder.
For How do I change the overall colors (background and font)?
For MAC : goto Sublime text -> Preferences -> color scheme