Greets,
Thanks for the comments and suggestions everyone. Especially to Caarrie for taking the time to post screenshots of the untested windows. I've made some modifications to the art, and updated the download links on my original post.
I hope you all enjoy it.

The overlapping of the top menu (or toolbar as named in gui files) is a known issue. PS gui focuses the windows even from transparent areas. Here even more so when most of the toolbar is transparent. There are two 'fixes' that can help: One can move the toolbar to the bottom of the screen like seen in the new
screenshot 1024x768. That doesn't fix the problem, but minimizes the effects. The other solution is what Prolix mentioned: to disable the popups alltogether. That way the whole toolbar window can be scaled smaller, avoiding the overlapping problem. I added an additional copy of imagelist.xml to the GP_skin.zip, that should do the trick. Just rename the imagelist_nopopup.xml to imagelist.xml. Then log in PS and scale the toolbar to your liking.
I've also made some changes to the backdrops, using Caarries screenshots as guide. Exchange window will still have a black background, since it's using "Standard Background" - an image also used by many other gui widgets. So it's impossible to make a specific backdrop for it, without editing window xml... and I'm not going there.

I did add the "Active Magic Background" resource though, but I'm not going to make a specific background for that either, not before it's layout is finished and known.
Finally, I forgot to mention that there is also a custom target marker included in the zip. It doesn't install automatically, as markers aren't really part of gui, but I thought it would make a nice change. To apply it, go to GP_skin.zip/effects/ and copy marker02.dds to your PS/art/effects directory. Be sure to backup the existing file. To see a sneak-peak of the new marker, see
screenshot 800x600.
To Caarrie:
Thanks for the screenshots, I've updated gm, quest reward and guild permission window backdrops. For the setup program mess I can do little; it uses examine window backdrop, but doesn't seem to take into account the texturerect-setting. In-game examine window looks ok though. The inventory doll shade is intentional. I tried to create an illusion that there is a slightly coloured glass on top of the char. And also to lessen the contrast between the doll and the window bg, so it wouldn't jump out so much. The equip slot icons are also left out intentionally, to make the inventory screen less messy. There's still the tooltip to tell what slot is what.
To Prolix:
The login and char creation screens are in PS/art/skins/base/client_base.zip, so they can't be changed with traditional skins. On my first skin, the elves one, I did create unique backgrounds for the char creation and loading screens too. But since they're not distributed with the client, I doubt anyone uses them anymore. Thanks for the combat - label tip, I've changed it to status.
To peeg:
Hmm, don't know. Haven't talked to the devs. If people like it, I've no problem donating it to AB. But for now, this was just an evening project for me.
To Tuxide:
LigH is ultimate the man! \o/
Later guys,
- Cherppow