His thumb's a lighter, that is a perfectly normal thing that Strange is not questioning. Except it isn't helpful in the slightest, as Strange just groans slightly, before shaking his head.

"I'm afraid it doesn't. We need more light and we need it to continuously burn. I'm not going to let you keep your thumb lit for the next who knows how long. We need candles," he repeats, insistently, trying to get Steven to understand this much.

Though Steven doesn't realize it, this is Norrell's library in Norrell's house. Ergo, Norrell would know where the candles are so he was send off to find them while Strange kept the candle from being snuffed out by the darkness. And the darkness in question moves towards Steven, covering some of the broken mirror shards as it inches closer towards him.