Gave it another try, this time replying YES.

And it did seem to install and run ok. But I didn't know what to respond to its questions about "default themes locations". I'm guessing it would have figured that out automatically in a "supported OS" like Win7. So I just ignored that questions.

Then, I figured out where to find the Application Background color item (in advanced appearance), and sure enough it had a value of "171 171 171" (lavendar) which was same value I'd already seen using REGEDIT, for the "AppWorkspace" item in HKCU\Control Panel\Colors and that I'd tried to change earlier to "0 0 0" (black) through REGEDIT but had no success.

It accepted my change, but I didn't know what to do next. How/where to "apply"? Whatever I did next seemed to prompt me to probably put it into my "default themes location" which I hadn't specified earlier. So I just saved it somewhere, honestly expecting it to simultaneously BE APPLIED. But it didn't seem to "apply", even though it did get saved where I put it.

I then got back into REGEDIT and the value I'd changed with Desktop Architect still showed the original "171 171 171" and the Application Background was still lavendar. Seemed like nothing was happening the way I wanted it to.

It then dawned on me that maybe I'd really just needed to restart (or perhaps just log off and back on again) in order to have picked up my very original change to "0 0 0" for AppWorkspace using REGEDIT in the first place!! Maybe it's only then that I really would have seen the effect of my change, and not "instantaneously".

So that's what I did. Used REGEDIT to change AppWorksace to the desired "0 0 0" in HKCU\Control Panel\Colors:

and then I restarted Windows.

And now... sure enough, my "application background" IS BLACK as I'd desired all along!

So, the tutorial pointed by Brink in the first post of this thread (which, by the way, I'd actually also found myself prior to coming here) is in fact CORRECT AND COMPLETE (although I wish the equivalance of the registry key of "AppWorkspace" to the old "application background" had been stated explicitly). What was missing was the need to then LOG OFF/ON or restart Windows, in order to then see the effect of whatever changes you've made actually take effect.

Ergo: Desktop Architect may actually do the same thing in a more intuitive (possibly for some) GUI way, but what's really happening here is a GUI-based interface to the same "advanced appearance" registry entries that REGEDIT is touching when you go to HKCU\Control Panel\Colors. Too bad MS has removed this dialog.

CASE CLOSED! Black "application background" now, and I'm happy.