I boot from SD. When I tried somthing in theme.xml and got a reboot loop, I simply took the card out, mouted it on a linux pc and reverted to the old file. Takes less than minute.
Now I remember that I also got a reboot loop when trying to define tasknavigator as panel_expandable instead of panel_box.
I also found that the status bar is hardcoded to have a maximum of seven applets in one panel, no matter how wide you make it.