Why not build some lamps? With the Enlighten Mod you are encouraged to build them because darkness will make your pawns slower.
Enlighten Mod features
With the Enlighten Mod light does matter. You must walk carefully in a place full of darkness. Now so do your pawns. They move slower in darkness. In contrast, bright places provide nice working environment. Manipulation will go 20% in brightly lit areas.
Bright environment needs a 0.9 glow, in vanilla only the sun and the sunlamps are able to glow that much. The sunlight from 8 am to 5 pm is at 1 glow. It is recommended that you download some light mods as well. Dark environment is below 0.3 glow.
These settings are modificable in the file EnlightenDef.xml
Enlighten Mod Review
Ever thought it was unrealistic that your pawns could work on a completely dark environment? The Enlighten Mod makes light actually matter, so you pawns will now require proper illumination to do their work.