I've had crested geckos, their tanks were never 100% dark. They won't breed if it is 100% dark. They were in the living room and the light from the windows never hurt them. A friend of mine breeds them and they are on a timer with plant lights above them because they are in planted terrariums.
Here is an example from Pangea:
Temperature, heating, and lighting.
Temperatures for crested geckos should be maintained between 72 and 80 degrees for most of the year. At temperatures of 85 degrees or warmer, crested geckos will become stressed, which could lead to illness or death. Cresteds can tolerate night time temperature drops down into the mid 60's but it is not necessary to provide this type of night time drop.
A two month cooling period is recommended to allow breeding crested geckos to rest. During this period temperatures should be kept at 65 to 70 degrees.
A photo period of 12 to 14 hours of light is appropriate for most of the year, with ten hours of light being appropriate during the cooling period. Lighting is most easily achieved with the use of fluorescent lights placed directly on the cage top. This will facilitate both the requirements of the geckos and any live plants within the enclosure should you choose to have them. It is unnecessary to use UVB lighting for crested geckos. For large collections consider lighting the entire room with natural or artificial light. Crested Geckos may cease breeding and laying eggs if they are given less than 12 hours of light.
In most situations room temperature is adequate for crested geckos, as long as the temperature stays within 70 to 80 degrees. If you are attempting to breed your Crested Geckos, temps should be kept between 75 and 78 degrees. Use a good digital thermometer with a temperature probe to monitor the environment. If temps cannot be kept in this range, a nocturnal blue or red heat light can be suspended above the cage for 24 hour heat. This type of light also allows for nocturnal viewing. Crested Geckos are not disturbed by this wavelength of light so it will not interrupt their photoperiod.