The bug

The fog inside of underground caves and other underground regions uses the color of fog which is seen aboveground. It probably shouldn't be visible underground at all, since it not only seems weird for caves, but also allows the time of day to be seen while underground, which doesn't make much sense.

How to reproduce

1. Enter a sufficiently large underground region larger than the current render distance
2. Note the very out-of-place fog

Expected result

The fog would be dark since the caves themselves also are - they're completely cut off from the sky in most cases.

Actual result

The fog matches the fog seen aboveground, which isn't desirable for reasons detailed above.

How to fix

Since biome json files can change the fog color of their associated biomes, the lush caves, dripstone caves and upcoming deep dark biomes could have their fog color set to something dark and different from the aboveground fog.

As for other underground caves not associated with one of these biomes, a generic cave biome "minecraft:caves" could be added, which also has a dark fog color.

This solution would also fix other issues such as MC-214830 (since there'd be no interruption from aboveground biomes anymore) and MC-214813 (since sky color can be customized by biome jsons just the same). It could also allow for the fixing of MC-118103, as this hardcoded sky darkening would no longer be necessary if the biome can cause it rather than the Y level.