From the ReadMe:

Cava Obscura (or "Darkened Caves") darkens all the dungeons in the game by scaling down the ambient lighting and fog brightness to 10% of the vanilla levels. This brings a great improvement to immersion, and makes torches, light spells and nighteye useful, even necessary, when exploring dungeons.

Unlike similar mods (Darker Dungeons, Let There Be Darkness, etc.), it does not totally remove this ambient light, though unless you have very good night vision and a better contrast display than I do, any difference is not noticeable.

For those wondering why I bothered to edit the lighting values of every single dungeon in vanilla Oblivion, the Shivering Isles and many popular mods by hand, rather than relatively painlessly set everything to zero, the reason is that Bethesda took the care to light those dungeons with a variety of different lighting themes. I wanted to preserve at last some of the information that the lighting represents, so that it could be used by other mods even when the dungeons had been darkened: for example, so that a shader could raise the lighting levels again, etc. In addition, the game doesn't handle total darkness as well as near-total darkness, and even a tiny level of ambient light can help make scenes feel a lot more dynamic, as the light from any light sources diffuses better.

A few dungeons have not been altered, as they were already at similar light levels to that of the altered dungeons, and if they were also reduced to 10%, their lighting would have been set to zero.

As mentioned above, there are optional plugins that will darken the dungeons of the Shivering Isles and many popular mods. The supported mods are:

A Brotherhood Renewed
Adense Epic Dungeon
AFK Weye
Arrows Of The Ayleid King
Daedric Shrine Ruins
Defenders Of Cyrodiil
Dungeons Of Blackwood
Dwemer Ruins
Elsweyr: Deserts Of Anequina
Glenvar Castle
Haunted House (by Kevy)
Haunted House (by WillieSea)
Heart Of The Dead
Hoarfrost Castle
Knights Of The Nine
Kragenir's Death Quest
Kvatch Rebuilt
Lost Sword Of the Aylieds
Mehrunes Razor
Mimics!
Oscuro's Oblivion Overhaul
Skyrim Nord Tombs
Spirit's Edge
The Ayleid Steps
The Forgotten Realm
The Lost Spires
The Northern Front
The Old Crow Inn
Thievery In The Imperial City
Thieves Den
UL: Arrius Creek
UL: Colovian Highlands
UL: Beaches Of Cyrodiil
UL: Brena River Ravine
UL: Blackwood Forest
UL: Skingrad Outskirts
UL: Snowdale
UL: Cliffs of Anvil
Verona House: Bloodlines
Windfall

This mod support plugin is a "Filter Patch". It requires Wrye Bash to function, as although it has many master files, Wrye Bash can filter out everything relating to mods you don't have installed, so that you can use it even if you have only one of the supported mods installed. See the 'Installation' section for more details.