It was explained in one of the "inside the episode" videos. Previously Winterfell stuff has taken place pre-Winter and been lit with natural light and prop candle-lights. They didn't want the same aesthetic or something
"Although there's no official word yet from HBO or the show's producers, it would seem this low-resolution presentation of the battle was an intentional choice. Robert McLachlan, a cinematographer on "Game of Thrones," seemed to back up that theory, telling
the website Insider that in the later seasons the show's producers wanted to try "to be as naturalistic as possible" when it came to lighting the sets. "
'Game of Thrones' looked really dark on Sunday, and we don't just mean the plot
Edit: This article blames the darkness on people not having their sets calibrated. Mine has been professionally colour calibrated, so I know the colours of my set matches as much as possible what the film-makers intended. My biggest gripe was the compression/pixellation.
“People don’t know how to tune their TVs properly.”
www.vulture.com