Fortunately, your juvenile offense doesn't count as a "conviction," so you are not facing a felony charge for petty theft. A petty theft with a prior conviction can be charged as a felony, if the person served jail time on the prior conviction.
Since the theft was under $50, the District Attorney may decide to charge you with an infraction, which does not carry jail time.
If the District Attorney decides to pursue the case as a misdemeanor, you could be facing as much as six months in jail. However, first offenders rarely get the maximum, especially when the theft was so small. In some cases, the judge will impose one day in jail -- just to make sure the offense qualifies as a "prior" to turn any future petty theft into a felony -- but you just report to the Sheriff's Department for a "book and release," where you are photographed, fingerprinted and released after a couple of hours.
Of course, every case is different, and nobody can predict the outcome of your case without a complete review of all of the evidence. If you can't afford to hire a lawyer to represent you, you are entitled to one at government expense (for a misdemeanor; if it's an infraction, you have to hire a lawyer or represent yourself) and the judge will appoint the Public Defender when you go to court.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.
Even if you hadn't done a diversion program, a juvenile case isn't a "conviction" - it's technically an "adjudication." Either way - since you complete the diversion program, it's not a prior.
Even with the low dollar amount, in Orange County, they typically file the misdemeanor against you, rather than the infraction. With the prior incident, even though it was as a juvenile and was eventually dismissed, it will definitely make the case lean toward filing as a misdemeanor as well.
The goal for you is to avoid an adult conviction for theft it at all possible. There can be factual as well as legal defenses to the charges, but it depends on the details. What can happen also depends on the details as set forth in the police report and what your attorney is able to work out with the prosecutor.