Going into the save folder, in the cfg folder... should see a file named...
GFXSettings.cfg
Should be able to just hmmm... delete it, which would revert all the settings in its domain, back to stock settings, I'd figure...
Could possibly... edit from another mod set of the same caliber, that is known, to NOT have the issues with it being dark in & outside of the sub.
Hope that info helps.
M. M.