What’s a poor rock band to do when your last album sold 8 million copies at a time when nobody buys CDs anymore? You can hire a guy who produced AC/DC, Def Leppard and Shania Twain albums that sold even more. On Dark Horse, “Mutt” Lange lightens Nickelback’s dreary post-grunge plod, applying guitar shimmer to prom ballads and detonating big beats under frat-party shouts and raplike vocal parts. Lyrics revel in dorkitude, hair-metal style: “No class/No taste/No shirt/’N shitfaced.” The two liveliest songs celebrate getting wasted with the bros; the most melodramatic laments a friend’s overdose. Lange keeps things rolling — and to his credit, Chad Kroeger gratifyingly comes off as more of a regular guy than a rock star.