Chances are good I’ve been using a Mac longer than anyone you know (late-March, 1984; a 128k Mac) but I don’t bother much with personal customization (not that Apple gives us many options to customize OS X anyway). I arrange app icons in the Dock a specific way. I set the Desktop wallpaper to cycle through whatever photos or images Apple provides in the latest OS X version (they’re obscured by open app windows anyway), and, well– that’s about it.
For Mac users who want a little more customization, there’s a free utility called LiteIcon which swaps out the Mac’s system icons so you can use those of your own creation, or others available on the web.
Open LiteIcon on your Mac and you’re greeted with a window that displays System Icons; those found in Folders, the Dock, and other Devices, as well as Applications. Click on Folders and you’ll see an array of Apple’s default folder icons.
To change an icon, simply drag and drop the new icon onto the folder whose icon you want to change. You’ll see the folder icon change immediately to the new icon. To get rid of the new icon, simply drag it off the folder icon.
Most Mac users will appreciate the personal customization option in LiteIcon, but probably don’t have any icons available to replace those Apple provides. In this case, Google is your friend and a search of “OS X folder icons” will yield plenty of choices, including these from AppStorm, and dozens more from PepSized.
Of course, you can swap out folder icons the old fashioned way. Apple has a step-by-step support document, but you’re likely to find LiteIcon to be much easier and faster. Is there a caveat to free? Possibly. So far, LiteIcon has problems with early versions of OS X El Capitan thanks to new security policies Apple has implemented, but it works fine on Yosemite.