First of all make sure that you customized error pages in proper way.

I recommend you to read this article.

First of all make sure that you customized error pages in proper way.

Follow simple steps:

1. Go to /errors and create new folder for example custom.

2. Copy /errors/default content into /errors/custom.

3. Edit /errors/design.xml and set custom skin

   <config>
       <skin>default</skin>
   </config>
   

Now customize html files in /errors/custom end styles in /errors/custom/styles.css.

If you want to access your styles through browser http://localhost/errors/custom/styles.css

Also I recommend you to read this article.