You need root access to your phones files. If you plan on modifying other parts of the phone, the easiest and prob most convenient way is to use WinScp on your PC and SSH setup on the N900. Google if you need directions.

Once you can see your root directory you want to replace the icons from this directory:

/usr/share/icons/hicolor/64x64/hildon

If you drag and drop the icons from my post they should just replace the original so best to first make a copy of the whole folder named "hildon". This way you can go back to originals when you get bored/change your mind etc.

Let me know if you need further clarification.