Well, I think you're trying to do different things together....
switch-themes is meant to change the plymouth GFXboot theme (google for example "pclinuxos switch-themes")...
=> that's the animation during boot (normally the bull that appears....: there exists an application for doing that ("ezswitch") and you can do a search for 'plymouth' in Synaptic to get extra themes for that)
More examples:
http://www.pclinuxos.com/forum/index.php?topic=76205.0Now your KDE Desktop background (the problem you have at the moment)...:
It's not necessary to change the whole KDE theme, just the background. See if the following is possible:
Right click on the Desktop and select "Folder View Settings"; then select "Wallpaper" (normally automatically selected) and change your background (you have there as many options as you want...)