The opening scene is presented in the dark of night, at midnight. Bernardo tells us that "'tis now struck twelve." "'Tis bitter cold," we learn from Francisco. The guards discuss "this thing...this dreaded sight," the ghost that has appeared to them twice already. The ghost does appear and looks to them like the assassinated king. The guards contemplate that this ghost is a portent, an indication of political strife to come. The apparition fades "upon the crowing of the cock," as daylight approaches.
That darkness of night only seeks to add to the darkness of the mood. Not only is the mood dark, so it the setting.
The ghost represents an omen, and further continues this fore foreshadowing.
I would say it begins fairly dark already. The play opens with a question: "Who's there?" already setting down the mistrust, and probably bringing tension as well. This could be the first signs we see of the corruption and mistrust and the questioning continues throughout the text.