Hello Sherri,
Currently this file is not editable due to the fact these styles are used specifically for the admin panel, and we do not allow you to make changes to the appearance of that section.
If you want to customize your invoices you do have a few option in Orders >> Order HTML/Email System, Order Invoice editor. You could simply put in the overriding css in this section and it may work for you. Otherwise, you could use DataPort to export your orders and use Mail Merge, from MS Word, to create your own template for invoices and you can have them look how ever you would like.
Also using Mail Merge you can print out all of the exported orders at once. For more information on Mail merge checkout -
http://kb.monstercommerce.com/Software/Dataport/dataport.aspJoe