Background

“Backgrounds” are additional character-building options that establish where your character came from and what kind of life they lived before joining the guild and may even relate to how they presently live their lives. While the main purpose of a background is establishing a framework for roleplay “flavor” as we like to call it, backgrounds additionally provide extra proficiencies, occasionally items and wealth, and “mini feats” – handy traits your character can use typically meant for use with social encounters and scenarios. 
There are no restrictions on character backgrounds in Chromatia. As long as it’s coming from an authorized source book, it’s free game for use. Likewise, you’re free to use the Custom Background options presented by TCoE to make something a little more custom fit for your character by mixing and matching traits to your liking.
Be aware that some backstories are specific to the adventures they come from, such as the Strixhaven or MTG backgrounds. If you choose a background that references specific, named places or organizations in D&D literature, check with staff for any relevant lore substitutions (for example, Strixhaven exists within the Chromatian multiverse, but on a wholly separate plane of existence and second to the Scrollbinder’s College).
As far as the story details of your backstory go, keep in mind you're given more or less full creative freedom to do as you please (within reason). Read The Astral Spark to help with mental framing the call to adventure if you haven't already, and check out I Know a Guy for constraints of what you can or can't make with with your characters and backstory.