Update 16-10-2012: Note this procedure is only valid in ultimate editions, link http://www.zoringroup.com/forum/viewtopic.php?f=6&t=2199#p9034 :(
It is very simple & looks elegant indeed.
Following are the steps on Zorin OS (I have tested on version 4 & 5, I hope it is correct for other versions)
- Step 1
- Right click on the panel at bottom & select 'Properties'
- Click 'Background' tab
- Select 'Solid color'
- Change the style to select transparency as required
- Step 2
- Right click on the Desktop empty area & select 'Change Desktop Background'
- Click 'Theme' tab
- Click 'Customize'
- Select 'Controls' tab
- Click 'Zorin OS Mac'