There is a way to do this using the CompizConfig Settings Manager (CCSM), although it is a bit more complicated.
You can very easily break your desktop using CCSM. Follow these instructions and any other answers recommending CCSM at
your own risk. If you run into problems, see this question for
instructions on how to restore Unity to working condition.
You can install CCSM by looking for it in the Ubuntu Software Center or by entering in a terminal:
sudo apt-get install compizconfig-settings-manager
Once installed, start it up and go into the options for the Opacity, Brightness and Saturation plugin. Click the checkbox to the left of it to enable the plugin first, then add a new rule under the Window specific settings section of the Opacity tab. The "Windows" textbox should contain:
...and the "Window values" slider should be set to 90 (recommended). This value can be tweaked to your taste.
For more detailed information and screenshots, see my (source).