Just editing the theme which is located here:
/usr/share/themes/beta/images (beta is the orange one and alpha the blue one)

For the LED and vibration pattern edit the MCE.ini located here:
/etc/mce

How to do that is well explaned many times in this forum, so I give you links to threads where you should find everything relevant to do this.

Acess your N900:
http://talk.maemo.org/showthread.php...ghlight=winscp

Any kind of mods:
http://talk.maemo.org/showthread.php?t=39565

LED and vibration patterns Wiki:
http://wiki.maemo.org/LED_patterns