Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Changing The Mac OS X Icons
Looking at the same graphics and icons can get boring at times. So if you want to change the way your Mac looks, here are the steps that will let you customize it. However note that you will have to download a free program called LiteIcon to be able to follow this tutorial.
Changing Application Icons
• Identify the application icon that you want to change for example Safari icon.
• If the icon is on the dock, right click it and move on to ‘Options’. Click show in finder.
• If the icon is not in the Dock or the desktop, then right click and select ‘Get Info’
• Once you are on the info box, ensure that all permissions are set to read and write. This is one crucial step and don’t forget this.
• Now, select the picture of the icon in the upper left hand corner.
• Find the icon you want replaced and copy that. You can search online for some great icons.
• Ensure that icon you selected earlier in the left hand corner is still selected.
• Now move on to Edit and paste the image. The icon will change. If it doesn’t, re-launch finder and log back to get the changes in effect.
How to Change Finder and Trash Icons
• For this you need to download LiteIcon.
• Now launch the application.
• Click on the tab that reads Dock.
• Identify the icon you want replaced. This application works for changing the Finder or the Trash’s icon. Remember for Trash you will need to select two images: one for full, other one for empty one.
• Select the images you want to use for the icons and drag them. For this simply drop the new icons in the box where you see the regular icons.
• Now click ‘Apply changes’.
• Password protect it.
• Click on Finder to re-launch it.
If you are not able to achieve the results, ensure that you are following the steps properly.