well i have been thinking because the underdark is well....dark, shouldn't there be some form of modification to these skills, like +1 to hide checks or something of that nature.

I believe they are allready built into the game as a +2 hide bonus while in shadowed or dark areas.

Game Mechanics Changes

Stealth is more effective in some areas (such as dark caves) than it is in brightly lit rooms.

kk, just makeing sure