Version:            (using KDE 4.1.1)
OS:                Linux
Installed from:    Ubuntu Packages

I came up with this idea when I saw the comment, "Plasma themes can't look good with light desktops." So, I backed up my current plasma-appletsrc and plasmarc files, then sought to change my desktop so it looked good with a light desktop - which it did in a short while. Unfortunately, the windows were still dark, so I changed the colour theme to the default which is lighter. Of course, using this system meant that I'd have to replace plasma-appletsrc and plasmarc every time I wanted to change from light to dark, then go into system settings to change the windows from light to dark (and vice versa).
So here's the punch line: how about an area in system settings' Appearance section where you can save all the current appearance configuration as a profile, then switch between profiles when you feel like it? It could have its own tab in the Appearance section that comes up by default and have a, "default" profile which is set to look like the desktop/colour scheme when you first got KDE 4.1.

I'm not sure how difficult something like this would be to implement, as... well, I'm just starting to learn PHP 5, if you know what I mean >.<