How do I customize unity on Ubuntu 13.04?

I find Unity Tweak Tool very powerful read here: http://lifehacker.com/5982844/unity-tweak-tool-customizes-every-inch-of-your-ubuntu-desktop

Open up a terminal and do as follows:

sudo add-apt-repository ppa:freyja-dev/unity-tweak-tool-daily
sudo apt-get update && sudo apt-get install unity-tweak-tool

Also check out this theme call elementaryish which I think is exactly what you are looking for: http://www.noobslab.com/2013/04/elementaryish-theme-for-ubuntu.html

You can also create a .themes directory in your home folder so you can extract and test theme sets without installing them to your root system, also make sure that the directory containing the gtk-2.0 or gtk-3.0 directory is the one copied into your theme folder...

Installing Tweak Ubuntu can make this easier also as it contains an option to install themes for you...