g) Why apple does not give an option to customize a desktop, colors, fonts and so on? ...... may be I missing something here
Because people have no taste and pick ugly gawdy UI elements. Just look at the atrocity of WinAMP skins on the Windows platforms. When people change the UI, it becomes a nightmare for support people and makes the OS look bad to others.
That said, there are utilities that you can search out if you really want to do that.
POWER!