I find the styling confusing, and find myself un-upvoting things on here when trying to upvote them. The button is muted when it is upvoted, but I expect it to be muted when "off", un-upvoted.

My suggestion is, make it muted when un-upvoted. Make it the bright green when it's been upvoted, to show I've "added" it or "turned on my vote."