Sunset Valley works a bit differently from the EP/ Store worlds.
Sunset Valley's sea, sky and lighting is governed by files in the game install directory- the
default settings if you will. Riverview uses these too.
The later store worlds, and the worlds that came with the EPs, all have 'colour ramp' image files in them that show a colour gradient for specific elements (shadow colour, sky colour, water colour, etc.) and ini files (text files) with numbers that control duration or frequency for certain aspects. These 'colour ramp' and ini files override the basegame default colours, but only for the world that they are in.
So- to remove the sea, sky and lighting settings for Midnight Hollow, open it in
S3PE and delete the following, if they are there:
S3_00B2D882_0076A684_7529C26EE8E2A9E6%%+_IMG.dds
S3_00B2D882_0076A684_28C841C9462BCDE0%%+_IMG.dds
S3_00B2D882_0076A684_E422CDDE7FE1F25F%%+_IMG.dds
S3_00B2D882_0076A684_ECCBBCB773C02131%%+_IMG.dds
S3_00B2D882_0076A684_F0A86F660985BF20%%+_IMG.dds
S3_1F886EAD_00000000_560C0FD7012DA7F3%%+_INI.ini
S3_1F886EAD_00000000_967BC6C3B3808C00%%+_INI.ini
S3_1F886EAD_00000000_FE1F6A95A24A604A%%+_INI.ini
S3_1F886EAD_00000000_2C02B3532B64EB49%%+_INI.ini
S3_1F886EAD_00000000_8DEA7AE7631A026A%%+_INI.ini
S3_1F886EAD_00000000_3EC0CF0CC4A6540F%%+_INI.ini
S3_1F886EAD_00000000_5E4F8E7B226066CA%%+_INI.ini
S3_1F886EAD_00000000_5E20253AF53E517F%%+_INI.ini
S3_1F886EAD_00000000_D89F9D186B7BB372%%+_INI.ini
You can see from
this image that those numbers are not what they look like in
S3PE exactly. What you are looking for is the long number which is the instance- you can sort numerically on that column and find the one you are looking for, for example 7529C26EE8E2A9E6 which is the first on my list above, and you can see that one in the image too in the middle of the five highlighted numbers.
So delete all those on my list above, and save the world file, and then when you play it the world should use the default settings and look a little brighter.
If you use any lighting override mods, these should show up in the world too (whereas ordinarily they would be masked by the world's own lighting settings).
If you ever decide that you want Midnight Hollow's original skies back, you can get the files here:
https://modthesims.info/showthread.php?t=512923
Sorry that thread was such a mess, it was really a research topic- I've never really written a proper tutorial!
I hope this helps!