[General] How can I work about Lighting in HD well?

[FlorianGeyerGiebelstadt]

The purpose of this question is to give visual impression of exploring deep and dark caverns to player.
First I tried the function " call SetDayNightModels("", "") " experimentally, the terrain got too dark and the lightings on the units got weird. It makes me feel like I am looking at the works before shader and rendering in 3d model programs.
I want to imitate atmospheres of Tomb of Sargeras in CSW Tide of Darkness, but I don't know how do I this.
I need a detailed explanation and guidance on this work.