If someone wants to make this into a Chrome Extension, that would be cool. I realize the code is terrible; I just threw it together in dev tools.

  var rainbow_parens = function(jq_el) {
    var html = jq_el.html();
    var processed_html = "";
    var nesting_level = -160;
    var color;
    for (var i = 0; i < html.length; i++) {
      if (html[i] == '(') {
        nesting_level += 160;
        color = 'rgba(' + Math.min(Math.max((nesting_level % 510) - 255, 0), 255) + ',0,' + Math.min((nesting_level % 510), 255) + ',1)';
        processed_html += '<span style="color: ' + color + '; text-shadow: -1px 0px 0px ' + color + '">(</span>';
      } else if (html[i] == ')') {
        color = 'rgba(' + Math.min(Math.max((nesting_level % 510) - 255, 0), 255) + ',0,' + Math.min((nesting_level % 510), 255) + ',1)';
        processed_html += '<span style="color: ' + color + '; text-shadow: 1px 0px 0px ' + color + '">)</span>';
        nesting_level -= 160;
      } else {
        processed_html += html[i];
      }
    }
    jq_el.html(processed_html);
  };rainbow_parens($('.code-body'));