Thorin did hear the sound of them joining him on the porch and he smiled slightly when he saw that Bilbo was home and safe. He also let out a sigh of relief and went to stand from his chair. But then he noticed that Bilbo wasn't alone, and the smile disappeared from his face. It was one thing to have to be cordial with Legolas, but to have the elf in his own home wasn't something that had been prepared for.
But he also didn't want to be rude, so he took a deep breath and nodded his head slightly in understanding to the explanation as to why Legolas was here. Whatever their differences, Thorin was grateful that Legolas had made sure that Bilbo made it home safely. After all, Thorin's priorities had changed since the last time Thorin had seen Legolas. So, it would make sense that his feelings might have changed at least slightly.
"I appreciate that kindness" he replied, nodding his head in Legolas' direction.