In simple terms it is accessory to the event (criminal act of stealing), there should be evidence to support this for you to get charged. I am not a lawyer but this is from my understanding, and are you an employee or just a customer who saw the event?
What, it anything, you can (that's "can", not "will") be charged with
depends a lot on the surrounding facts and circumstances.
For one thing, assuming that you are not actually in on it,
you don't have any duty at law to tell shop staff if you see somebody stealing.
Nor do you have any duty to confront the thief yourself.
But, for the sake of discussion, and for the benefit of people who read this in future,
let's assume that you were (in some way, however minor) "in on it".
First things first - did you actually get caught?
If so, caught by whom (shop security? somebody else?)?
If you were not caught, then do the police know about you, or about your involvement?
I ask because sometimes quite often, police threaten to charge somebody with being an accessory
as a way to bully them into giving evidence against the actual offender.
Let's say next, that they do know about you.
To found an accessory charge, the police would have to be able to prove
that you knew (or reasonably could have suspected) that the other person had committed the offence.
Further, what might (not will, might) happen depends on your relationship (if any) with the offender.
Even if they are friends, just because a Billy is present when Bobby commits an offence
does not automatically implicate Billy in that offence, even as an accessory
(perhaps you've heard the term "innocent bystander").
It can also unfold differently if you are a minor (somebody aged 10-18),
and differently again if you actually got caught.
It can also depend on what the person stole.
Knocking off a two dollar packet of Twisties is one thing.
Stealing a grand's worth of cigarettes is something else.