You can change it in Preferences>Interface>Appearance>Color Theme>Light. Do it with no documents open so it becomes the default.
The new dark UI was introduced almost three years ago and as already noted is very easy to change. The size of the interface will depend on the resolution of your monitor.
There are scaling settings available in your preferences as well.
Thanks, yes, good to know it's in prefs. I have been using CC but using CS6 because I have an older Mountain Lion machine (for various good reasons). I also have a Yosemite machine that uses the newer versions of CC apps but I dont often design on that one because it is smaller. I had noticed the grey background in newer Illustrator a while back and didnt care much because i dont use it as much, but I use INDD every day. Somehow the latest CC got downloaded onto my Mt Lion machine and I opened it and was greeted by the grey background. it didnt occur to me it would be a pref, and I am very glad they made it one.