How to reskin
So, a quick and dirty way to do UI reskin: get standard UI module (from .myp using EasyMYP + warhammer hash), edit the textures however you can (e.g. Photoshop + NVIDIA Texture Tools), add the module to the game as a <Replace/>ment.
Why this may not be a cool way to reskin
The downsides: those are full replacements of default modules, not just new addons, therefore they include more things than just a new skin.
They will conflict with other addons that replace standard UI modules in order to fix things. (I know only one mod that does replace a default module - fixgit_scenariogroupwindow. The module has no textures though.)
If the module gets updated, you will absolutely have to update it too. (Thankfully, the game is dead so they won't be updated too often.)
They can be kinda buggy to manage from ingame so you'll have to place/remove actual folders with skins to/from "/Interface/Addons/" folder. Maybe something else but, well, that's the simplest way to do reskins I could think of.
If someone salvages YakUI reskin part to be a standalone, it'd be really nice too.
Example
Now here's the ugly part, the example skin. I selected a couple of modules that are visible the most and just applied a standart PS filter to them making sure that it's painful to look at. You can redraw most of the UI - make buttons rectangular, implement material design, add more shrooms, make it Windows 95 or just MS Paint doodle style etc.
Spoiler:
Just unpack into /AddOns/ folder. Preferably into a subfolder so you won't have to search for every included module when you decide to delete it. It shouldn't cause any after-effects, so feel free to check it out.
Problems
At least one part didn't work - mastery trainer backgrounds are still loaded from default module, fixing this would require modifying .lua/.xml. There are also other some small things - you may have noticed that some modules are Loaded(!) with a warning - that's because not all files are actually included in hash for EasyMYP and I was too lazy to look for them. You will have to look for missing dds/lua/xml files in corresponding folders, rename and put them in correct place yourself if you want.
Redrawing everything is a quite long process, i will be really happy to see any reskins people will make (if anyone at all bothers to).
Default modules links
Here are just those default modules that include textures for people who don't want to unpack .myp themselves
https://www.dropbox.com/s/tmu37exqlr2g8 ... es.7z?dl=0
Here's full standard interface including textured modules
https://www.dropbox.com/s/0vmr7doriaoq8 ... ce.7z?dl=0
