There's an option in the settings menu that lets you deactivate "class-based default outfit" appearance.
Unfortunately, that's not how it works, unless they've added a new option in the beta patch. The existing option ("Class-specific outfits for the party") just means that the class outfits are shown without the equipped armour. If you disable it, (it's by default disabled), it will instead show the characters with the class outfits
and the equipped armor.
There's a mod that lets you change the class-specific armour (to get rid of the damn backpacks and stuff), but as far as I know, that's the only way to do so.