Promised patch that makes Kopete use chatwindow style names instead of style paths.
Few things are still todo: like figuring out whether global directories follow local ones as returned by KStandardDirs or vice-versa, and adapting the code to prefer local styles over global ones with the same name.

If you currently have some style set, I think you'll want to remove
[Appearance]
StylePath=/path/to/style

from your ~/.kde/config/kopeterc

or replace it with
[Appearance]
StyleName=Kopete (or whatever)