Alright, after messing about a little, I think I managed to solve at least part of the problem.
I figured the whole thing was related to the gnome power manager config. As I wrote before, I uninstalled and reinstalled it and the problem persisted.
I lurked about a little more, since a lot of people seem to get stuck with the same problem, and thought to do
sudo apt-get install --reinstall gnome-settings-daemon
I did it, restarted, and yay, I can change the color scheme and icons again. But the theme itself and mouse pointer are still not working.
I also get the same bulky icons, the same font, the same sort of display I was getting when stuck with the blue-and-gray theme. My external HD also does not open. I double click it, all icons vanish for a second, then come back, but the drive itself will not open up. I tried moving a file to it by dragging the file directly from desktop to the HD icon, I get the File Operations window, so I assume the HD is working, but not being displayed. The HD also shows up prettily in Disk Utility and it's not full.
On a desperate attempt to solve it, after lurking a while longer, I found two more hints: maybe my disk's full (from what I managed to see, it's not) and running
sudo dpkg --configure -a
was suggested as well. I'm still stumped.
Anyone else got a similar problem or experienced something like this?