Oh I've been wanting a tking interface for WoW for some time.
It is possible to edit the artwork the UI uses without writing an addon. It would be the equivalent of only editing the windowpieces01-etc.tga files in EQ1.
The WoW version of "uifiles\default" is interface.mpq. You can unpack that and then edit files and put them into the Interfaces folder for the UI to use instead of the default.
For instance, to change any of the default artwork:
1. Use this tool from Blizzard:
http://www.blizzard.com/support/wow/?id=aww01669p to extract all the artwork. (Definitely suggest backing up/renaming your current Interfaces directory as this will fill your current Interface directory with an unpacked "Interfaces" folder-- there are a bazillion files)
2. Then go to
http://www.cosmosui.org/vjeux/ and grab three utilities: BLP2 Viewer, BLP2 to TGA and TGA to BLP2
3. Using the BLP2 Viewer, browse the interface.mpq for the artwork you want to change:
4. Once found, go to the path in your Interface folder created from step 1. The artwork will be there with a .BLP extension
5. Convert the blp to tga with the util from step 2.
6. Edit the artwork in any graphics program that can save in 32-bit tga (or 24-bit color + alpha)
(God bless alpha channel. No need to stay in the lines!)
7. Convert the tga to blp with the util from step 2.
8. Put the edited file into it's proper place in the Interface folder and now WoW will use your artwork instead:
You can zip the single file to exist on its own, much like a uifiles\mycustomart in EQ1 could have simply a windowpieces01.tga and it would use the default for everything else.
Obviously tking's stuff had a lot more involved, like button placement and stuff. I wouldn't suggest any mod touch any of the FrameXML stuff unless it's for your own personal use (and even then the Glue stuff is off limits).
But a mod that creates a new visual look can definitely be done with artwork alone.