When it comes to customizing your Mac, there are many ways to do it. You can easily change Dock's appearance, apply various themes, use a custom wallpaper and more. You can either use specialized applications or make use of various standard Mac OS X features and built-in tools to customize your Mac. Another simple method that allows you to add a personal touch to your Mac is modifying the icons of your folders and apps. If you are planning to change the default icons on your Mac, then I will show you how easily it can be done.
Before we start, I'd like to note that the process of icon personalization is always the same, regardless of what your target is (folder, application, system volume or individual files). Now, I will show you how easy it is to perform this task, with a friendly step-by-step guide.
Obviously, in order to change the icons of your files, folders or whatever targets you have in mind, first of all you need to have a new icon ready for use. You can either create an icon using your favorite image editing tool, take screenshots that can be used as icons (for your desktop folder, for example) or download Mac icons from the Internet (the default format for Mac icons is .icns). If you are planning to download icons from the Web, there are lots of various websites with huge icon collections. Just type in "Mac icons" into the search field and you will find plenty of them.
Navigate to the folder where you have stored your images or downloaded icons.
Open the desired photo you want to use as an icon in Preview.
Copy the image to the clipboard (either by using the "Command + C" keyboard shortcut or the "Copy" option).
Go to the file, folder, application or volume of which you want to change the icon and select it.
Once selected, right click on it and click on the "Get Info" option (or use the “Command + I” keyboard shortcut).
Click on the icon in the upper-left corner of the opened window, next to the target's name (in this case, the blue folder icon as seen in the image below).
After clicking on the icon, you will notice that it now has a blue border around it. Simply paste the image from your clipboard in order to replace the default icon ("Command + V").
As you can see, changing the file, folder and application icons on your Mac is a very simple process. You can use any type of image you want as a replacement and, if you are not happy with the result, you can always change the icon again using another photo or even revert it to the original picture (go to the "Get Info" panel, select the icon and, instead of replacing it, simply press "Delete"). You can also use third-party applications, such as Folder Icon Changer or LiteIcon to personalize your icons.