There still are not too many themes for Grub that are are easy to install. The process of theming Grub is quite complicated. There is a guide available here: A Beginner's Guide to Theming GRUB2

Though that includes a 37 page PDF document to read through! A simple but effective way of sprucing up Grub is to add a custom background image. The easiest way I've found of adding one to Grub is to use Grub Customizer

To install this enter these commands in a Terminal:

sudo add-apt-repository ppa:danielrichter2007/grub-customizer

sudo apt-get update

sudo apt-get install grub-customizer

Then run Grub Customizer, when it has loaded click the Preferences button and then the Appearance tab.

See image below for newer version of Grub Customizer.

Then select the file icon under background image and navigate to your chosen image file (I found that selecting one of the default wallpapers looks effective, I'm using the new Pangolin image from the 12.04 wallpapers). You can also change the font colour of the menu at this point (useful if your background image colour makes the text unreadable - a preview will appear when you select a background image to check whether you'll have any colour clashes).

Then close the preferences pop-up and select the Save button on the main screen, then you can reboot your system to admire your handiwork!