Manage party and customize decks just shows you the default cards.

I understand after start, but not for a new character.

Sometimes the game allows you to customize, other times it doesn't. What's the secret handshake? For the life of me I cannot figure out why following the same steps does not work.

No one else in party.

Customising only before the first scenario. If you play any scenarios, you Are not allowed to custom the (whole) deck.

I would Also start completely new croup. Not sure what happens is you add new character to old croup...

This is for a new character when the entire group has fallen.

I found the best way is quit to main menu, quit the app.

Re-launch app.

Start new campaign.

Start new character.

Customize cards.

Unload character.

Quit/restart.

Reload save game.

Add new character.  

Cumbersome but it seems to work 

All of the characters seem to have a default starting deck. You don't really need to customize it from the start (I think overall the choices for starting deck were pretty good).

 

I don't remember ever getting the option to fully customize the character decks when starting the game, and after playing numerous scenarios I was definitely able to get better stuff into the decks than just cards from the basic set (which is all you should have access to when first creating your deck anyway).