I havent seen the first 4 films in a while. And I only saw the third one once (hated it), but I do really enjoy the first 2 and TMNT. But mostly when I was younger I watched the 2 animated series.
I dont remember the movies being extremely "light" in tone. But not extremely dark.
ANd I think you can have a dark story and explore darker things in a lighter movie. Personally I wouldnt mind a tone a little darker than the latest Sherlock Holmes movie. Cool action, funny interactions and characters, dark villain/dark case
More than the tone Im worried about how the Turtles look in live action. Ive always been more drawn to the animated versions (TMNT included)than the live action ones even as a kid because the suits looked bad to me and the action in the animated one was always better to me