"It is dark in the evening."
くらく is the dictionary form of the verb for "dark". くるい is the adjective form of "dark".
When you state that something is NOT something, you use the dictionary form + 〜ない. (Note: this makes it an adjective.) But this is a casual use. To make it polite, use です after it.
くるい is the positive adjective form of "dark". くるくない is the negative adjective form meaning "not dark".
To state that something IS something, use the adjective form.
今日はくるい。It is dark today (but this is impolite).
To make it polite, use です.
今日はくるいです。It is dark today (polite way).
To make it negative, use the ない form. 今日はくるくない。
To make it polite use です. 今日はくるくないです。
ゆうべ（夕べ）"Evening": Seems to be mostly used to refer to last night/yesterday evening, and in which case is written 昨夜 (lit. Yesterday Night).
ゆうがた（夕方）"Evening": Seems to be used for "evening" in general, though it refers to the dusk period where the sky changes colour before night （よる「夜」) properly.
こんばん（今晩） / こんや（今夜）"This evening/night"/"Tonight": Apparently interchangeable for specifically this night.
This is what I have gathered from various comments/sites, hopefully someone more knowledgeable can come by and clarify if this is accurate enough or complete rubbish...