First of all download the plasma theme, in my case the Air theme from http://kde-look.org/.
The download link can be http://download637.mediafire.com/emdmm4mrmwmg/4d0ztq24wmf/air.tar.bz2.
Extract it to .~/.kde4/share/apps/desktoptheme/.
That's it. Change to your new theme.
