Does anyone know if there’s a way to share the preorder character themes, backgrounds and frames with my party? I can share all my purchased content and home brew but not these apparently

Here's how you can accomplish sharing the pre-order bonuses:

1. Create a campaign.
2. In the campaign, create an unassigned character.
3. Set the unassigned character's theme, background, etc... to whatever your party member desires.
4. Once finished, have your party member join the campaign and claim the character.

Your party member will keep the theme, background, etc..., but will be unable to further modify them.

If your party member already has a character made:

1. Create a campaign.
2. Your party member joins the campaign with the character.
3. Your party member Unassigns from the campaign.
4. You Claim the character.
5. You set the theme, background, etc...
6. Once finished, you Unassign from the campaign.
7. Your party member joins the campaign and claims the character.