We know that working as a private detective has made Marlowe disillusioned by all the corruption he sees, so it's no surprise that the tone of the novel is primarily dark and pessimistic. Let's take a look at the opening of Chapter 6, a classic example of Marlowe's voice. He's staking out Geiger's bookstore, waiting to see if and when Geiger makes an appearance:
Rain filled the gutters and splashed knee-high off the sidewalk. Big cops in slickers that shone like gun barrels had a lot of fun carrying giggling girls across the bad places. The rain drummed hard on the roof of the car and the burbank top began to leak. A pool of water formed on the floorboards for me to keep my feet in. It was too early in the fall for that kind of rain. I struggled into a trench coat and made a dash for the nearest drugstore and bought myself a pint of whiskey. Back in the car I used enough of it to keep warm and interested. I was long overparked, but the cops were too busy carrying girls and blowing whistles to bother about that. (6.1)
This passage has many of the typical characteristics we expect to find in a Chandler novel: rain, booze, sketchy cops. The tone in this passage is definitely gloomy, and Marlowe doesn't sound like he's having such a great time.
Plus, check out the unusual word choice he uses as he takes a drink of his whiskey: he drinks just enough to "keep warm and interested." The "warm" part makes sense since he's probably cold and wet sitting inside his rain-soaked car. But why "interested"? How would numbing his senses with alcohol help keep Marlowe interested in what is happening at Geiger's store?
It's a classic example of Marlowe's cynicism. He sees so much corruption while on the job that alcohol becomes a way for him to cope with his disillusionment, to stay "interested" when he mostly just feels disgusted. And of course Marlowe's characteristic dark cynicism also comes through in his portrayal of the cops, who are too busy flirting with giggling girls to notice that he has been overparked.