A modified css.php file to allow for easier editing of the stylesheets when working on a MyBB theme. Bypasses the cached files and uses *.css files you can edit in your editor of choice, and instantly see results.

css.php

<?php
/**
 * MyBB 1.8
 * Copyright 2014 MyBB Group, All Rights Reserved
 *
 * Website: http://www.mybb.com
 * License: http://www.mybb.com/about/license
 *
 */
/**
 * Modified css.php from MyBB 1.8.6, by github.com/fabd
 *
 * The purpose of this hack is to allow easier editing while developing a MyBB
 * theme (on a local MyBB installation).
 *
 * Edit the FLAT_FILES_PATH below to point to a subdirectory of the MyBB
 * installation. Any css files found there that match the name of a stylesheet
 * referenced by the current page, will be returned instead of the cached file.
 *
 * If your theme folder does not contain a file matching the requested css
 * file, then the file is either pulled from the cache/ folder as it normally
 * is or it is pulled from the database (when MyBB creates css.php?id= urls).
 *
 * A special comment is inserted at the top of the CSS files to help you figure
 * out if a css file is not found, or is not named properly, or is missing from
 * the cache altogether (usually if the cache/ folder is empty, load up
 * global.css in the admin and Save it once to regenerate the cache).
 *
 * Setup:
 *
 * 1. create a subfolder
 *    $ mkdir theme
 *    $ chmod 777 theme
 *
 * 2. add this redirect rule to the .htaccess, which enables to bypass the
 *    cache/ folder
 *
 *    RewriteRule ^(.*)\.css$   /css.php?file=$1 [L]
 *
 * 3. create the css files in the theme/ folder, to get started with the
 *    current theme, just copy/paste the stylesheet contents from the 
 *    Templates & Style section in the Admin.
 *
 * 4. when your theme is complete, simply copy/paste the contents of the css
 *    files back into the admin, export the theme and import it in your
 *    production forum.
 *
 *
 * Suggestion:
 *
 *   Create a git repo, a "theme" branch that contains this modified css.php
 *   file and your css edits. That way you can checkout into your master
 *   branch so that you can keep this modified css file and the theme files
 *   in a special branch, which will not be deployed in production.
 *
 */

define("IN_MYBB", 1);
define("NO_ONLINE", 1);
define('THIS_SCRIPT', 'css.php');

require_once "./inc/init.php";
require_once MYBB_ROOT . $config['admin_dir'] . '/inc/functions_themes.php';

// chose a subdirectory where we edit stylesheets
define('FLAT_FILES_PATH', MYBB_ROOT.'theme/');

function exit_with_http_404($message = '')
{
  header("HTTP/1.0 404 Not Found");
  echo $message ? '(css.php): '.$message : '';
  exit;
}

function _theme_file_real_path($baseName)
{
  return FLAT_FILES_PATH.$baseName;
}

// return contents of a css file if present in our theme folder, otherwise false
function _theme_file_get_contents($baseName)
{
  return file_get_contents(_theme_file_real_path($baseName));
}

$stylesheet_comment = '';

// if 'file' parameter is present, this is a redirect from .htaccess
if ($fileName = $mybb->get_input('file', MyBB::INPUT_STRING))
{
  $fileName = MYBB_ROOT.$fileName.'.css';
  $baseName = basename($fileName);

  // use our flat file if it is found in our theme folder
  if (false !== ($stylesheet = _theme_file_get_contents($baseName)))
  {
    // help us figure out what is happening
    $stylesheet_comment = sprintf('/* (css.php) Using stylesheet "%s" found in FLAT_FILES_PATH */', $baseName);
  }
  // otherwise return the cache file as is
  else if (false !== ($stylesheet = file_get_contents($fileName)))
  {
    $stylesheet_comment =  sprintf('/* (css.php) Stylesheet not found in FLAT_FILES_PATH, using cached "%s" */', $fileName);
  }
  else
  {
    exit_with_http_404(sprintf('(css.php) Not found in FLAT_FILES_PATH, and no cached file for "%s"', $fileName));
  }
}
// this part is when MyBB explicitly uses css.php with a theme stylesheet id
else if ($sid = $mybb->get_input('stylesheet', MyBB::INPUT_INT))
{
  // grab the stylesheet name from its id, and its contents in case we bypass the flat file
  $query = $db->simple_select("themestylesheets", "name, stylesheet", "sid=".$sid, $options);

  if ($data = $db->fetch_array($query))
  {
	  $baseName = $data['name'];
	}
  else
  {
    exit_with_http_404(sprintf("Invalid stylesheet id '%s'", $sid));
  }

  // should we use the flat file, or the contents from the database?

  if (false !== ($stylesheet = _theme_file_get_contents($baseName)))
  {
    $stylesheet_comment = sprintf('/* (css.php) Using "%s" */', _theme_file_real_path($baseName));
  }
  else
  {
    $options = array(
      "limit" => 1
    );
    //$query = $db->simple_select("themestylesheets", "stylesheet", "sid=".$stylesheet, $options);
    //$stylesheet = $db->fetch_field($query, "stylesheet");
    $stylesheet = $data['stylesheet'];

    $plugins->run_hooks("css_start");

    if(!empty($mybb->settings['minifycss']))
    {
      $stylesheet = minify_stylesheet($stylesheet);
    }

    $plugins->run_hooks("css_end");

    $stylesheet_comment =  sprintf('/* (css.php) Stylesheet not found in FLAT_FILES_PATH, using "%s" from database */', $baseName);
  }

}

header("Content-type: text/css");
echo $stylesheet_comment."\n\n".$stylesheet;

As explained in the file comments, you'll need to add this rule to the .htaccess file:

RewriteRule ^(.*)\.css$   /css.php?file=$1 [L]