The Close Combat Situational Modifiers table on p.89 lists fighting in:
Partial Darkness: -20%
Darkness: -40%
Fighting Blind or in Pitch Black: -60%
Torches and lanterns list a radius of illumination. What I do is say it is normal light out to the radius, partial dark out to 2x the radius, and darkness out to 3x the radius (if in pitch black).
Nightime would be Darkness or Pitch Black depending on the sky. Keep in mind that in 2nd Age Glorantha there is no visible moon so night is Pitch Black all the time (the Blue Moon is generally not visible and the Red Moon doesn't exist yet).
Night Sight sees in darkness without penaly, dark sight works in Pitch Black.