Previously Installed Packages Won't Display standard Theme

[MR_NINJA]

Hi, so when I installed mint on my old lappy, I got the cinnamon edition, not knowing which I would prefer. Later, I wanted to switch to KDE, so I tried, had some troubles, and aborted. I tried again a few days ago, and got it working. However, when I open up my previously installed packages, the oxygen theme doesn't work properly. It is rounded around the edges and has the blue border, but inside is all sharp edges and rather win98ish.

Note that it is Mint 13, and all on the same system, as I didn't just do a fresh reinstall.

Thanks,

MR_NINJA