gmail.monospace.coffee.js

###
// ==UserScript==
// @name          Gmail Monospace
// @namespace     http://owns.ch
// @description   Make emails and textareas in Gmail monospace
// @author        Alen Mujezinovic & David Anguita
// @homepage      http://gist.github.com/4536975
// @match         http://mail.google.com/*
// @match         https://mail.google.com/*
// ==/UserScript==
###

css =
"""
td > div, div.Bk, div.editable, textarea {
  font-family: Menlo, Monaco, DejaVu Sans Mono, Courier, monospace !important;
}
"""

try
  # Prepare style tag
  node = document.createElement "style"
  node.type = "text/css"
  node.appendChild document.createTextNode css

  # Append to all heads
  heads = document.getElementsByTagName 'head'

  head.appendChild(node) for head in heads

catch err
  try
    console.log "Error applying GMail style: "
    console.log err
    console.log err.stack
  catch err
    # pass

gmail.monospace.user.js

// Generated by CoffeeScript 1.3.1

/*
// ==UserScript==
// @name          Gmail Monospace
// @namespace     http://owns.ch
// @description   Make emails and textareas in Gmail monospace
// @author        Alen Mujezinovic & David Anguita
// @homepage      http://gist.github.com/4536975
// @match         http://mail.google.com/*
// @match         https://mail.google.com/*
// ==/UserScript==
*/


(function() {
  var css, head, heads, node, _i, _len;

  css = "td > div, div.Bk, div.editable, textarea {\n  font-family: Menlo, Monaco, DejaVu Sans Mono, Courier, monospace !important;\n}";

  try {
    node = document.createElement("style");
    node.type = "text/css";
    node.appendChild(document.createTextNode(css));
    heads = document.getElementsByTagName('head');
    for (_i = 0, _len = heads.length; _i < _len; _i++) {
      head = heads[_i];
      head.appendChild(node);
    }
  } catch (err) {
    try {
      console.log("Error applying GMail style: ");
      console.log(err);
      console.log(err.stack);
    } catch (err) {

    }
  }

}).call(this);

Install UserScript in Google Chrome:

1. Open chrome://extensions
2. Drag *.user.js file
3. Allow extension