So many seem to have a problem with this as a typical English sentence. It never even crossed my mind till I started reading all theses negative comments. I've lived in the contiguous 48 all my life I see nothing wrong with it's dark today. I think some just like complaining.
Give me a break
True, but I think it depends where you are. Here in England I usually hear 'It's dark out today', or 'It's suddenly gone dark'. If someone said It's dark today my first reaction would be to ask what is, unless the meaning was clear by pointing to the outside or something.
Accents have at least two uses in Spanish.
First, it tells you which syllable gets stated with more force--"accented".
Second, it can also change the meaning of words.
For, example, "Tú" means "You" while "Tu" means "Your". The same for "Mí" means "Me" while "Mi" means "My". The same for "Él" means "He" while "El" means "The".
The list is too too long to go through; so, watch for the accents.
Mila, that's essentially it. The verb ser is used to talk about characteristics, and estar is used to talk about conditions. So if "being dark" is something typical or defining of the thing you're talking about, you'll use ser. If not, estar is the better choice.
El cielo es muy oscuro aquí. - The sky is very dark here. (a typical occurrence for this place)
El cielo está muy oscuro hoy. - The sky is very dark today. (it's something out of the ordinary)
"It's dark today" is not a sentence that makes sense in English. It is overcast, it is cloudy - those make sense and are common usage. Duo has an obligation to teach in a way that makes sense in either language. Maybe in Spanish "Está oscuro hoy" would be correct usage, but their translation is not common usage - generally if it's dark, it's night, not day - therefore they should accept translations that would be commonly used in English.