Murky, Somber, Sinister, and Foreboding
Macbeth opens with three witches conjuring on a heath amidst thunder, lightening, "fog and filthy air" and then proceed to throw around sinister prophesies, so, yeah, we're going to go ahead and say that this is one dark and foreboding play.
Even the humor is bleak. In Act II, Scene iii, the Porter tells some knock-knock jokes (seriously) about who could be knocking at the doors of Macbeth's castle at such an hour. He goes through an extensive comedy routine as he imagines what it would be like to be the porter at the gates of hell. (Pretty busy, apparently, because there's so much evil in the world. Then again, notes the Porter, Macbeth's castle is much too "cold" to be hell.) The joke, of course, is that Macbeth's castle is a lot like hell, especially since Macbeth has just murdered Duncan while the king was sleeping.
Cue the uncomfortable laughter.
Later in the play, Macduff's young son and his wife crack jokes about how Lady Macduff can just trot off to the market to buy twenty more husbands, since hers has apparently abandoned her (4.2). And just as we're enjoying a few chuckles, a couple of murderers enter and stab Macduff's son in the guts. So there's that.