EDIT Ok, masta just pointed out to me that this does not work 100%, the skin is loaded but the GUI points arent correct, you will infact have to wait until they update the gui file to point to the path of the folder and not just the default directory.
This is actually fairly simple and I have tested it twice to make sure it works.
18.1 brought us "Added support for custom guis outside the 'Guis\Default' directory (we still don't give any customer support on third party GUIs though)." The problem is that the old way of doing it was to over write the default GUI, this will no longer happen as EACH gui will be its own entity in your folders now.
Step 1. Download your GUI. For this write up I am using Notum Dovvetech Dark Here
Step 2. Open the zip file, you will see a Folder called CD_image
Step 3. Click on GUI then Default.
Step 4. Open a new windows explorer and go to \Funcom\Anarchy Online\cd_image\gui
Step 5. Make a new folder in \Funcom\Anarchy Online\cd_image\gui called Dark
Step 6. Open the Dark folder and copy the contents from the "Default" directory from the zipfile into the \Funcom\Anarchy Online\cd_image\gui\Dark folder
Step 7. Go back to the \Funcom\Anarchy Online\cd_image\ folder and go to Textures.
Step 8. Go to \Funcom\Anarchy Online\cd_image\textures\archives
Step 9. Copy the folder named Dark4 from the Dark-4.18.0a-1.zip\cd_image\textures\Archives to the \Funcom\Anarchy Online\cd_image\textures\archives
Step 10. Launch Anarchy Online.
Step 11. Click on settings -> GUI
Step 12. Click SET DEFAULT
Step 13. Click Change, then direct it to Funcom\Anarchy Online\cd_image\textures\archives\Dark4\GUIGFX.UVG I
And thats it, you can have more then 1 skin now for your game.
!NOTE! From now on when you start AO there will be an error message, just click OK until this is fixed in future patches.