Load domain-specific CSS file on a multisite
It's kinda messy, but you might be able to do something like the following in a custom module:

function mymodule_preprocess_html() {
  $site = $_SERVER['HTTP_HOST'];

  switch ($site) {
    case 'siteurl1':
      //Add your CSS for site 1 here with drupal_add_css
      break;
    case 'siteurl2':
      //Add your CSS for site 2 here with drupal_add_css
      break;
    case 'siteurl3':
      //Add your CSS for site 3 here with drupal_add_css
      break;

  }
}


Or if you specifically want it in your theme files you could do the same in template.php

function mytheme_preprocess_html(&amp;$variables) {
  $site = $_SERVER['HTTP_HOST'];

  switch ($site) {
    case 'siteurl1':
      //Add your CSS for site 1 here with drupal_add_css
      break;
    case 'siteurl2':
      //Add your CSS for site 2 here with drupal_add_css
      break;
    case 'siteurl3':
      //Add your CSS for site 3 here with drupal_add_css
      break;

  }
}


To be honest I think you could use hook_preprocess_page in the module as well.

Many ways to skin a cat and all that jazz.