Dock in Mac OS X helps us to reach important items and applications easily. You can customize dock in Mac OS X to suit your needs. You can add or remove items from the dock, change its looks and animation behavior, etc., easily. You can change icons displayed in docs so you can select your favorite icons to represent applications. Here are some simple steps to customize your dock.
1) Go to /System/Library/core services/
2) Now right-click on dock and select show package contents from the menu.
3) In this window go to Contents/Resources
4) Select the files scurve-s.png, scurve-m.png, scurve-i.png, scurve-xl.png and frontline.png and backup these files.
5) Now you can delete above mentioned files to replace them with your modified files.
6) Open Terminal and type “killall Dock” without quotes and hit enter. Your new dock should be ready.
You can also customize Dock Indicator and separator graphics
To do this select the indicator_small.png, indicator_medium.png, indicator_large.png, Indicator_small.png and indicator_medium_simple.png files and make a backup of these files.
Now you can replace these files with your modified png files to change its look. You can also change the icons for your trash by replacing trashempty.png and trashfull.png files with your modified files.
Note: Backup original files that you want to replace with your modified files. This can help you restore dock if something goes wrong.