Move the mouse over the display and click on it/them. Once the 2D display pops up, move the mouse pointer to the bottom right area of the panel. Now use the Mouse Wheel to dim the display.
With my mouse wheel set to 3 line (the default setting), it takes one move to dim it so the background is black, while two moves makes it even darker.
In the attached image, the red box on the 2D Display is the area I used the mouse wheel. The left view of the display panels is after 1 (one) move, the right view of the display panels is after 2 (two) moves.