k 1st download nokia pc suite (its the 1 ive beeen usin for long)
then connect via usb
then a ballon should pop up sayin 5800 conected via usb if it doesnt come then there coulb be somethin wrong with usb connection..
then open pc suite then just drag the theme file (sis) and click the tick mark..
then on the phone install it to ur mem card or internal mem
and then go to settings , personal , themes , general then find ur theme and select it
hope i helped & gud luck.