Normally styles and decorations come with a configure script so you normally just compile it like any other program. "./configure && make install" (maybe adding a --prefix to the configure command).
If the tgz is just a cpp file then, personally, I wouldn't thrust is very much.
The window decoration is Plastik but the theme I don't know.
In mandrake you update the system with the software management tool (or urpmi). Just find a site that supports mandrake and as a kde repository, add it as a new package source and update.
