Minimum required for a working UI
 

A friend of mine was talking about how she wants a different targeting ring, BUT doesn't like having to deal with custom UIs and keeping them updated and whatnot. So I got to thinking about it, and someone please correct me if I'm wrong, but if a custom UI doesn't have a particular file (such as the persona window) the Default one will simply be used, right?
What is the bare minimum required to be in a custom UI folder for it to function, but still HOPEFULLY use almost everything from the Default UI, and thus always be up to date?

Only the file you like to replace, everything else will be default.

If you just want it to work, only include the files that you're changing. Any files it doesn't find are automatically pulled from default.

If you want it to show up properly in /loadui (which is still buggy, fwiw) when not using any other custom UI, you need to include an eq2ui_skininfo.xml. You can copy it from default and just change the name and such in the data tag to whatever you like.

So technically I could make a folder called MyDefault, and put ring_target.dds and ring_target_con.dds in the images sub-folder. This would give me the Default UI EXCEPT for the targeting ring? After, of course, telling EQ2 to use MyDefault for the UI.

It should, yes. You'll just have to set the folder in your EQ2.ini since it might not show up properly in /loadui (but /loadui can still be buggy anyway, so setting it manually is preferred).

If you look at SOE's Default_Blue, you'll see basically the same thing. They only include what is different in that version: things in the images folder, the ColorStyles file and the skininfo file so that folks can find it properly in /loadui.