Learn Arabic (Modern Standard): السماء مظلمة الليلة.

The word for sky, السماء, is one of a few Arabic nouns that are feminine by convention, even though they don't end in the typical feminine marker (taa marbuuta). Because السماء is feminine, the adjective describing it must also be feminine, which is why we use مظلمة instead of the masculine مظلم.

Where is the word for "is" in this sentence?

Arabic doesn't use a present tense verb for "to be" (like "is" or "are"). This is called a nominal sentence. You simply place the definite subject السماء (the sky) right next to the indefinite adjective مظلمة (dark) to create the meaning "the sky is dark".

Does الليلة mean "the night" or specifically "tonight"?

Literally, الليلة means "the night". However, when used like this to show when a state or action happens, it functions as an adverb meaning "tonight". In fully pronounced formal Arabic, it would have an "a" sound at the end to show it is acting as a time reference.

How is مظلمة (dark) related to other words like ظلم (injustice)?

They share the same three-letter root: ظ-ل-م. Words from this root generally relate to literal darkness or moral darkness (doing wrong). For example, ظلام means darkness, and ظلم means injustice or oppression. مظلمة is an adjective form describing something that is currently characterized by darkness.