Try giving the them soaked oats and germinated grains. Latter will provide many if not most benefits of consuming summer time greens. Temperature you are describing since still wet should not be all that stressful so long as nutrition is good.My babies continue to avoid their coop during daylight hours, except to lay. But some do seem to have lost quite a lot of weight. I have chosen to not add light or heat, and they have a protected and covered area as well as a large back yard. This has been the harshest winter I recall in the 17+ years I've lived in SW WA. Now that the severe cold has broken for a while, it is very wet. And the large grassed area is mostly just mud now. Almost no greens available for them to nibble on. I feed them organic layer pellets (and the chicks are on organic chick starter), along with a varied home-made non-gmo scratch a couple of times a day, and chopped up leftovers of my summer and autumn harvests.
So - as this is my first winter with my chickie-babes, I am just watchfully waiting and looking forward to winter being over. It's rather stressful, though! And I sense that they are somewhat stressed too.