There literally is not any flexibility. The rules for customizing your background aren’t in the PHB. You cannot, by the rules we have, play an Acolyte with a boost to strength, or with the savage attacker feat, or thieves’ tools. The new backgrounds were sold to us as custom by default with a selection of premade examples if you don’t want to choose all the elements individually, with the freedom to change out whatever you want from an example. But what they actually gave us is fixed backgrounds (“oh but you can change the description if you want to!

) with no rules for customizing them even in the player’s handbook at all. I think it’s pretty reasonable to be annoyed that they delivered the literal opposite of what got over 70% approval in the playtest.