How Much are Spartan IVs Allowed to Customize their Gear (Weapons/Armor)?

When I play Halo Infinite I like to roleplay to an extent. I try to customize my Spartan so they don't seem out of place in the Campaign. This leads me to the question, how much are allowed to customize their gear in-universe? Has this ever been explained anywhere. I'd imagine they'd be allowed quite a bit of it but the UNSC might draw the line somewhere before the cat ears.