In no way should JLA be dark and gritty... In the beginning.
You have dark and gritty batman in the corner a little pissed off that all these camp idiots are wasting their god like powers, then a 1/4 the way into the movie Superman gets his ass handed to him, and the rest of them suddenly realize that it's not just about bullying bank robbers robbers anymore.
Darkseid has (literally) crucified Superman.
It's battle they will probably lose that they have to be brave to wade into, and it's the end of the world either way.