I'd say that Nova and Apex are probably the two most popular on devices running 4.0+. I use Nova Launcher myself and absolutely love it.
To change individual icons in the dock (at least in Nova), long press on the one you want to change and select Edit, then tap on the icon image itself. You'll get a window with a variety of options. An easier thing to do would be to download a compatible theme from the Play Store (there are literally thousands), and select the theme from within the launcher's settings menu. This will change the look of all the icons on your device, and some themes include multiple options for each app (although you'll probably have to change them individually if you want the alternates).
Have fun!