It looks as though the layout.php isn't used for the authoring and administration suites. I wanted my modified styles/layout also applied to this part of the site, but I had to edit :
to make sure everything was allright (in fact I grepped and then replaced everything so I'm not sure all this files have to be changed, but I'm quite sure the two first need to)
I was wondering if I missed something in the beginning (my philosophy : act first, ask questions after ). I also assume that each upgrade will overwrite my changes again ...
PS1 : I'm using v.0.6-pl2
PS2 : I changed the default theme, and after a while there were too many changes to track. So I build a new theme upon serendipity_v2. If you wanna take a look go to http://www.nussbrothers.net/seb/
If you wanna use the theme just go ahead and download it at http://www.nussbrothers.net/seb/res/template_nussi.zip
The picture on top comes from openphoto.net and the colors are inspired from zeldman.com
PS3 : Garvin is gonna kill me (3 days => 3 questions)