How to apply drupal_add_css by block_id in Drupal?
You can try this code. Paste this code in file template.php

function YOURTHEME_preprocess_block(&amp;$vars) {
  // use dpm($vars) to see struct of variable $vars
  $delta = $vars['block']-&gt;delta;
  switch ($delta) {
    case 'abc':
      //add css here
      break;
  }
}


Block ID are 


string 'block'
$variables['block']->module
$variables['block']->delta


You can check block delta to use drupal_add_css()