Learn Spanish (Latin America): El horizonte oscuro está muy lejos.

You would replace "El" with "Aquel" and say: Aquel horizonte oscuro está muy lejos. Because horizonte is masculine and singular, you use aquel. We use the aquel family of words specifically when something is physically or conceptually far away from both the speaker and the listener, which fits perfectly with something that is muy lejos (very far)!

A horizon is a permanent thing. Why do we use "está" instead of "es"?

In Spanish, we always use the verb estar to talk about physical location, no matter how permanent it is. Whether you are talking about where a person is right now, or where a mountain, a city, or the horizon is permanently located, you must use estar.

Horizonte ends in "-e". How do I know whether to use oscuro or oscura?

Nouns ending in "-e" can be either masculine or feminine, so their gender has to be memorized. In this sentence, the article el reminds you that horizonte is masculine. Because it is masculine, the adjective describing it must also take its masculine form, giving us oscuro.

If we were talking about two dark horizons, would lejos change to lejosos or something similar?