I know that light kills the resins and yeast during the fermentation process so as long as the fermenter is covered it should be alright.
I've never heard that light kills yeast. I'm 99.9% sure that's not true. Try as you might, I don't think you can "kill the resins" with light either.
The only issue with light and beer is blue, green, and UV light interacts with an chemical compound in hops to produce sulfurs that "skunk" the beer. I don't know scientifically how much it takes, but I ferment all of my beers in uncovered glass carboys in a spare bedroom with two windows with normal cheap blinds, and have never had an issue.
I can only deduce that exposure to indirect light for 4-6 weeks at a time is not enough to skunk beer, therefore your beer covered with a bag in the shower with the shades drawn is going to be just fine.