You can do this with a new tool that several users here on Ask Ubuntu have made. Its called Unity Tweak Tool. It's still in early development but it is very stable from my testing.
To install it:
sudo add-apt-repository ppa:freyja-dev/unity-tweak-tool-daily
sudo apt-get update && sudo apt-get install unity-tweak-tool
Now open the dash and search for "Unity Tweak Tool". It looks like this:
Open it up and navigate to Launcher.
From there you can:
Change the transparency of the launcher
Change the color of the laucnher
Auto-hide the launcher
Or change the background for applications (This is closest to what you want I think)
Another suggestion is changing your icon theme under Icons.
You need to note however, you can't very well change icons of other applications.
Brightness is relative, but I got them this dark:
For certain icons, like the software center, you might have to change the actual icon, wherever the system stores it.