Hi Johanna,

Il fait froid hier soir means It is cold last night. (hier soir means both yesterday evening and last night).

This is probably a typo and you meant Il a fait froid hier soir. - It was cold last night.

Both Il a fait froid hier soir and il faisait froid hier soir  can be tanslated as It was cold last night but with a very subtly different meanining.

For example if you got up in the morning and the lawn was covered in frost you would likely say Il faisait froid hier soir  (It was cold during the night) but if you got up and it was quite warm but you noticed that one of your prized  delicate plants had suffered frost damage during the night you would probably say Il fait a froid hier soir. (there was (has been) a cold snap last night).

I hope this explains the differences between the Imparfait and the Passé composé , in this case quite subtle.