Thanks for taking the time on that really informative reply. I see you've been away a while, so I'm not sure you've noticed - As of quite recently (since you were last here?), nearly all of the 'default' artwork is no longer written into the dol, but placed in a folder next to the boot.dol. I'm sure, as someone with an interest in these things, you immediately see the possibilities this invites. But as yet, no-one has contributed anything as a result.
A guide could run even shorter than your contribution above "replace the images in the folder with yours, matching the name and dimensions" - but it could also be as well done as
this guide on skinning the homebrew channel. Something similar for the wiiflow default theme, a couple of colour/texture variations maybe, might encourage the awaited influx.
As it was mamule called for the influx of (non-fantastic) additions to propose, created the default artwork, and has plugged away at the same skins for years, over and over again, creating a deep understanding of the process - I thought he was the ideal author. Sorry that's not to your liking - Once your up to speed, maybe you fancy doing it - and stringing a cohesive sentence together, or two?
Honestly, I wouldn't worry about him not working on his (truly excellent) themes - my guess is he's putting something together right now