what is the truth behind this: everyday exposure to PC will darken skin?

I think someone is teasing you. If your PC monitor was giving enough ultra-violet radiation off to darken your skin then it would be a VERY strange monitor. There is no way that your computer is generating anything that will darken your skin. Your PC generates a low level elctromagnetic field that might cause interference to nearby radio receivers, and old style 'CRT' onitors generate an electrostatic field that will attract dust. Years ago there were some concerns about pregnant women using such devices, but there was never anything proven, so there's not a worry there either. Generally - don't worry. Using your PC may give you backache, shoulder ache, sore wrists, itchy eyes - but you won't get a tan from it.