Never a dull moment when learning languages.
Formal pronouns do exist in Norwegian, but they're considered obsolete, and you'll only see them used in jest or in formal letters from international companies. They do not feature in this course.
De = formal singular you (subject form/nominative)
Dem = formal singular you (object form/accusative)
Deres = formal possessive for the singular you
Note the capitalisation.
Your secret's safe with me, Barrie, but you know you can't be that cheeky and get off without a lesson:
"Hun er varm."
"She is warm." (as in not cold)
"Hun er heit."
"She is hot." (as in attractive)
"Heit/het" could also be used to express that someone's too hot temperature-wise, overheated or running a fever.
We won't usually make a distinction, but it's possible to use "het", "kjempevarm", "veldig varm", "glovarm", "gloheit", etc.
For the above sentence, there wouldn't be any need to: If it's cold, then it's asking whether they're warm enough, and if it's warm, then it's asking if they're too warm.
For now, remember that the ending of a adjective matches the noun it modifies:
Huset er varmt. - The house is warm. (neuter)
Hytta er varm. - The cabin is warm. (this ending is for both m/f)
Barna er varme. - The children are warm. (plural)
Later in the course, you'll encounter additional rules for attributive adjectives that modify definite nouns and possessives.