"It is dark!"
Translation:Il fait sombre !
"C'est sombre" is incorrect, however it can be used like: "C'est une route sombre et dangereuse ..." which means It is a dark and dangerous journey ... , → the adjective here describes a journey that is precarious (so caution is advised).
"Il est sombre" means He is dark (gloomy, grave, serious etc) → the adjective here describes a mood.
Il fait sombre means "it is dark", as in there is little light available. However, the verb être can be used if further context is given.
Compare these two sentences:
"Cet appartement est trop sombre, ouvre les rideaux !"
This apartment is too dark, open the curtains!
"Il fait trop sombre, ouvre les rideaux !"
It is too dark, open the curtains!
As you can see, if sombre is used with a preceding noun, the verb être can be used, otherwise, if the structure is "It is.." then faire is the required verb.
According to WordRef : http://www.wordreference.com/fren/il%20fait%20noir
it is a valid translation.
Was it rejected? If it happens again, report it.
The program has sort through it's collection of answers for a word that fits with "c'est" and "foncé" is synonymous with "sombre". It is usually used in reference to a colour, eg "bleu foncé"= dark blue. Though, I am fairly certain, that you can't say "c'est foncé" in reference to the weather.