Mac: Apple doesn't give you many options for customising your dock from the System Preferences. Free app cDock lets you customise the colours and backgrounds to your heart's content.
cDock lets you change just about anything on the Dock. It comes pre-installed with coloured themes so your don't leave your Mac with the transparent default (I like the light blue background myself). How-To Geek explains how to add a border around the dock and do a lot more — which is handy if you're remote controlling another computer and not sure which dock you are using. Check out the link for all your customisation options.