Adjust the Lighting

At the moment the Sun Light is being applied to the generated map which does not give it a good look due to the shadows showing up from the walls.

Adjust Sun Light Intensity

For the Sun Light, set the Intensity property to 0.

Adjust Skylight Intensity

The Skylight Intensity may also be a bit strong, adjust this to make the map a little darker. To make this easier to test, you can enter player mode and pause the game when the map has been generated, then you can adjust the value to get the look you want. Just remember to copy the value when you are happy, and reapply that value when exiting preview mode.

Test the Game