I imagine it something like this:
examplemod:example_helmet;0.5
examplemod:example_chestplate;0.5
examplemod:example_leggings;0.5
examplemod:example_boots;0.5
wearing each individual armor piece would give you 0.5 temperature, and wearing the whole set would give you two. Using negative numbers will make it cooling instead.
Also, there should be a config to make it so armor protects you both from hot and cold. Let's say, if you put just the ID of the armor items and no values, it will protect you from temperature completely.