xmonad is configure by creating and editing the file:

    ~/.xmonad/xmonad.hs

which is a Haskell source file. Here is an example,

    --
    -- An example, simple ~/.xmonad/xmonad.hs file.
    -- It overrides a few basic settings, reusing all the other defaults,
    -- and also uses the DynamicLog extension.
    -- 

    import XMonad

    main = xmonad $ defaultConfig
        { borderWidth        = 2
        , normalBorderColor  = "#cccccc"
        , focusedBorderColor = "#cd8b00" }


You may typecheck this file, or develop it in ghci, as you see fit.

To have xmonad pick up your defaults, try mod-q. xmonad will attempt to
compile this file, and run it. If it is unable to, the defaults are
used.

The default configuration values are defined in the source file:

   XMonad/Config.hs 

the XConfig data structure itself is defined in:

    XMonad.hs