[xmonad] How to set which fonts and theme to use by editing files?

In Ubuntu we can do this by running the application called "gnome-appearance-properties", in which we can set the application font to be serif insteand of sans, etc. I'm planning to install Arch linux with Xmonad and no desktop environment. In that case, how do I set the fonts and theme? BTW, what files does the program "gnome-appearance-properties" access to and make changes? -- Sincerely, Yuliang Wang