I used Bigrock for reference in terms of lights and textures. I used the artificial light and spotlight levels from the mine area almost identically for my map. Although, I did utilize a bluer fog, and maybe intensified artificial lights to really get that deep blue feel.
Feel free to utilize the decompiled Bigrock map to help you on that.
It may be better to not have such a large area as I do on my mvm map. Feel free to utilize a beginning 1/4 of the map to emphasize the entering of the mine with the Payload cart, from an adjacent facility. Then you can keep the gameplay going through underground yards, that become progressively more constructed until you come to the underground mining facility (Utilizing the drill from cp_antiquity hanging above).
If you'd like a better reference of where you might want to take your map, here is an area I utilized to test my underground theme (Even before MVM and Bigrock existed, working more off of Portal 2 for lighting hints). Please don't use as much deathpit as I show here though, as that was before I considered how OP that made Pyro/FAN scout.
Also, if you'd like to see how profound an effect lighting can have underground, and that you have to be careful in not lighting the ceiling, here are some very contrasting pictures from the production of mvm_underground.