You can, but it doesn't have the same meaning. "It is light outside" doesn't mean that it's sunny. It means that is no longer night time ( or that it isn't night time yet and the sun hasn't gone down). For example, you might say "In northern Scotland in the summer, it's light until midnight." This doesn't mean that it's sunny - it just means that the sky isn't dark yet. Or you might say "It's light at 4 am in summertime", meaning that dawn comes very early.