Problem I can see the sky on the horizon while underground. Solution?

[Emblin]

I'm trying to make a modpack focused on living solely underground, in a large cave system (it's basically nether generation, but in the overworld). However, unless my view distance is absurdly high, I can see the night-time stars on the horizon. I need a way to either stop the sky from showing while underground, or some kind of mod or texture pack that turns the sky a flat black color.

**EDIT** I heard Optifine allows custom skies, but I am unable to use it in my modpack due to it's ridiculously long load times.