There are many ways to customize the appearance of Elementary OS. If you want to change the top panel on Elementary OS Luna, Wingpanel Slim is a good choice. Its similar to the default wingpanel but it slimmer. Also its more customizable via Elementary Tweaks.
Wingpanel Slim can be configured via Elementary Tweaks easily.
Ok if you are interested, here’s how to install Wingpanel Slim on Elementary OS Luna:
sudo apt-add-repository ppa:versable/elementary-update sudo apt-get update sudo apt-get install elementary-tweaks wingpanel-slim
Now you can play with wingpanel slim via Elementary Tweaks.