There's no way you can install themes or window decorations from "Configure Your Computer".
The right way is More Applications -> Configuration -> Configure Your Desktop.
There you have 2 options:Workspace appearance, for changing desktop themes, window decorations, cursors and your splash screen.
Application appearance, for changing color themes, fonts, icons.
You can get new by pressing the "Get new..." button in every category.
I'm guessing this is what you did...
You can also uninstall previously installed themes from there.
Of course, you can manually delete themes, icons and cursors by deleting their corresponding folders from your disk:System wide desktop themes:
/usr/share/apps/desktopthemeUser's desktop themes:
~/.kde4/share/apps/desktopthemeSystem wide icons and cursors:
/usr/share/icons/User's icons and cursors:
For more customization options have a look at: www.kde-look.org