Not sure when it changed, but when I played Minecraft before, setting the brightness up to Bright would typically oversaturate EVERYTHING. Darkness was no longer an issue when it came to "Bright". Caves would look like the overworld during the day.

Now I'm watching my brothers stream, and he has his brightness maxxed and he is STILL having issues seeing skeletons in the dark (granted his monitor isn't the best, but if I can see the darkness, then I'm wondering what happened).

Dark and "Bright" should not come into the same sentence. So can you restore the Brightness, or maybe create a new level of brightness, 'oversaturated', so there isn't a "dark" any more?