There are a few things you will need before being able to enable transparency for your dialog boxes and modal dialog boxes is compiz-fusion, which can be found in the repositories. Install it in a terminal with
"sudo apt-get install compiz"
*(without the quotes).
Then, install compiz-config-settings-manager with the command
sudo apt-get install compiz-config-settings-manager
If you want to be fancy and do it all in one command it would look like this:
sudo apt-get install compiz ; sudo apt-get install compiz-config-settings-manager
If all goes well, and your graphics card and drivers and memory are all able to handle desktop effects, compiz and compiz config settings manager should be installed.
Right click on the desktop and click "change desktop background".
Click the tab "desktop effects" and if it's not already marked, mark "normal".
The program should search for drivers and then enable desktop effects (if all is well with your graphics card capabilities).
Now to enable transparency for your dialog boxes and modal dialog boxes:
Click on the main Gnome menu -> system -> preferences ->compizconfig settings manager
This will open up the settings manager for Compiz (wow big surprise there). Scroll down to "opacity, brightness and saturation" and mark the box next to it to enable it. Then click the actual button to enter the opacity, brightness and transparency options.
Under "Window Specific Settings" click "new"
When the dialog box comes up, change the opacity to something other then 0 before entering in the criteria for which components you want to make transparent. It can be a pain if you somehow type "ccsm" into the windows field with it still set to 0, then try to somehow find the little slider thing to change it to something visible. Anyhow best practice in my opinion is to change it to something like 80% and then choose your windows.
So to choose the windows we're going to be listing them according to their window type.
After clicking "new" to enable transparency for dialogs, modal dialogs, popup menus and drop down meus you would enter
"type=Menu | PopupMenu | DropdownMenu | Dialog | ModalDialog" *(again, do not include the quotes).
I have mine set at 70, choose whatever you prefer. You can use this same process and add another entry with "type=any" set to 90 or 95%, which is usually visually pleasing.
Hope you enjoyed this tutorial, have questions or comments feel free to let fly
Keep it pimpin' out there...