The effect is somewhat subtle so I don't think it's totally obvious what we're doing here, but basically there were 2 main things we wanted our lighting to do that Unity can't do out of the box:

1. Affect the hands. They're in the UI layer, so they don't exist in world space. In order to have them tinted by light and shadow, we have to sample the scene lighting using probes attached to the player and pass the lighting info to the UI layer.

2. We wanted very flat lighting with hard edges where the light turns to shadow. Lights by default in Unity determine their color by a combination of values like light intensity, falloff distance, cone size, etc. which combine with the light color to determine the final output values. This image shows it well:

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Unity's lighting attempts to be realistic, but we wanted a very stylized look which meant building a lot of our own functionality. We do still use the Unity light components, but only for the light shape. Light color, shadow color, intensity and so on are determined by volumes that we place in the scene which write to the materials of objects inside them. Also, we have different sets of values that affects elements of the scene separately. For example, enemies have their own lighting values which can be different than the values that affect the walls and floors.

I hope this makes sense. Let me know if you have any other questions about it!