thehelpcenter:

You wanna have these lights at the top of your blog? It’s easy!

• First of all, you’ll need to go yo your customization page and click on html.
• In your CSS (probably at the beginning) You’ll find body{. 
• Find the background line and delete it.
• Remplace it by this code:

background: #YOURBGCOLOR url(‘http://img90.imageshack.us/img90/3152/lightsn.gif’) repeat-x top center;

This is it ;)

P.s.: They look better on your blog because the gif is bigger than 500.