Click the red square texture preview under the Texture area to open up the Lumas panel. Here, we set the color to pure white, disable all of the specular controls, and set the shading algorithm to Oren Nayar, with a roughness of 200 and Illumination of 150. This will ensure that the Lumas will be pure white where the object faces a light source, and black where it faces away from a light source.
The Colorizer shader that we have placed this Lumas shader in has an inverted gradient. This will invert the output of the Lumas shader so that it will be black facing the light source, and white facing away. We are then using the output from the Colorizer to multiply these black/white values with the city lights map. The result will be that the city lights will dim in areas where the object is lit, and only be at full strength where the object is in darkness. Perfect!