There are several ways in which the theme of light and dark is explored in Emile Zola's Naturalistic novel Germinal.
Given that the setting of the novel is built around multiple small villages and mines, the theme is most illuminated by the fact that life in the mine is spent in darkness and life above is spent in light.
Etienne Lantier is an outsider who comes into a village and begins to try to lead the mining people out of the oppressive workplace. One could look at Etienne as an image of light and the village and mine being one of dark.
The theme of light and dark is important given the mines rule the lives of the miners and their families. Without the mine, many of the villagers would be unable to survive--it is simply the most prominent livelihood of the village (and many of the surrounding villages).
Another way that the theme of light and dark is explored is through the promiscuous behaviors of some of the characters in the novel. Many of the sexual descriptions are given by describing the couples simply laying down, outside, at night and having sex. The sex, therefore, is masked by the night (and never referred to as happening during the daylight hours). The curious thing about this is the fact that the couples only hide in the dark and not in homes or other hidden areas. It is simply outside where all can hear.
One last example of the theme of light and dark in the novel is the fact that the mines are killing many of the workers. The darkness (the coal) leaves with the workers in their lungs. Hidden by being inside their bodies, the coal is still able to exist in dark. This shows that even some dark things can exist when brought into the light.