The problem is that your computer thinks it has touch capabilities, and because of this Discourse adds the touch and touch-discourse classe to <body>.
Replace them withno-touch and no-touch-discourse with your browser’s dev tools to test it: the rainbow effect should play on hover.
Most Discourse hover effects are probably absent from your computer because of this.
I have the same problem, and I don’t understand why my computer thinks it has touch capabilities. Perhaps because it’s connected (or was connected at some point?) to some device which “confers” a wrongly detected touch capability. 
Edit: Now if your device has a real touch support (like a touch-compatible laptop, or tablet…), thent it’s purely a feature, not really an issue 
Pretty sure you can re-enable the rainbow effect on touch-compatible devices with CSS on your instance.