The temperature in cabe houses is always the same, about 20 degrees Celsius,

(What is the relationship between 'the same' and ' about 20 degrees Celsius'?)

The phrase, "about 20 degrees Celsius" is an appositive describing "same."

Are they in apposition? Then 'same' is a pronoun or a noun?

Yes. They are in apposition. The "same" is used as a noun.

The phrase "the same" is used to convey that the temperature in all cave houses is consistent or uniform. In this context, it means that every cave house maintains a temperature of around 20 degrees Celsius. So, "the same" refers to the uniformity of the temperature, while "about 20 degrees Celsius" provides the specific value of the temperature.