Okay, that's a little confusing. Let me put it this way.

Overall, on a scale of 1 to 10, with 1 being pitch black and 10 being pure white, the world of INverse ranks about a 4 or 3 most of the time, but there are patches of 8 or even 9. For all the horror Michael inflicts upon the world, love, courage and good still break through the darkness often enough to keep hope alive in peoples hearts. So not every book will have a bleak ending, but that's not to say one can hope things will go good when they're bad. When it's Dark, it tends to stay Dark. The mood here is more of desperate hope in the face of a crushing evil, than it is despair.

Contrast is touchy. Among the Archangels and the Demon Princes, it's very high. Your Archangels are loving, understanding and (mostly) care for humanity even if their focus is on the War. The Demon Princes all tend to be truly evil and selfish beings, and like Michael, will use anyone and anything to achieve their goals. However, the contrast changes radically among the lesser demons and angels (although more so with demons). Angels, one and all, tend to be good and selfless beings, and very few wish to Fall... compared to the canon world, there are many fewer Falls. Demons, on the other hand, are a mixed bunch. Many demons can hear the call of the Symphony beyond their own selfish one, always pressing in, always singing them home... so many demons tend to either be quite gray, or as black as their Princes, with few in between. Redemption is a much more common occurrence, but few demons succeed in their run; Michael keeps a tight grip upon his resources, and Dominic is ever vigilant in his Condemnations, making for a lot of sad situations as demons are dragged to Hell to be torn apart and used as Force fuel. Angels who Fall are usually the demons who are black hearted.

As for humor, it depends on the GM. Kobal -is- the Archangel of Laughter here, and humor does exist... it's just not quite as common. When it comes onto the scene though, it's the kind of joy and laughter that brightens the soul right up. But INverse is not meant to be played as a Silly game. (Of course if you want to and can manage it, go right ahead. Michael would make a good "Curses! Foiled again!" type of bad guy, like Skeletor or something...)

You can modify INverse however you like, and it doesn't have to be as dark as it's default is. But it can be most fun to watch the players achieve victory after victory... bringing love and light into the lives of man... only to have it crushed as the endless legions of Michael infect the world like a deadly cancer.

And besides, who doesn't want to see a megalomaniac Michael wreak havoc on all that is Holy?