使用Javascript用if语句更改样式表？

I'm having trouble with my code.. Basically what I want is for when the volume slider is over halfway for it to turn red, and when its under halfway for it to turn green. I have the style sheets created I just don't know how to link it all together. Am I close? Thanks for feedback. (Focus: Last function and the Volume1 input)I'm having trouble with my code.. Basically wha