The expression still light outside has existed for long. And it is grammatical.
See: Different 'Light' Usages (1)
From the Harper's Magazine, Vol 149 
And now, with the window shuttered and the door closed, she could not tell when it was dark. She could pretend it was still light outside. But the village was quiet, quiet.
Sir John Alexander Hammerton The Masterpiece Library of Short Stories 
… up the slippery winding stairs. Then he sighed still more deeply and went in again. Though it was still light outside, the basement was clothed in a dark blue half-light, so that all that could be seen …
Stop the Clock Deborah Paul writing in the Indianapolis Monthly Mar 2007
When we turn the clocks forward this month, daylight will begin to extend longer and longer until summer comes and before we know it, it's nearly 10 p.m. and still light outside. This just isn't natural.
Ronald Schmidt, The Undelivered 
… it is now almost 10:00 pm. However, it was still light outside. He has never seen the so-called Midnight Sun before.
However, it seems more of a literary and colloquial use rather than formal.
Still light outside at 9pm (garvolinda on YouTube 0:45)
Has light in this sense gained greater acceptance and usage of late over bright?