It affects it - - it does not necessarily prevent it. First, you absolutely must disclose this on the I-485. If you fail to do so, it will be considered a material misrepresentation on your application and that might prevent the adjustment. Now as to the offense itself. The amount of the fine; the value stolen will not necessarily determine the impact of the conviction. The key is 1)What was the maximum sentence you could have been sentenced for the offense as well as the actual punishment imposed. It is highly likely - - but it depends on the particular state law - - that this will be a de minimis offense and no waiver would be needed. If it is not a de minimis offense, it will likely be considered the type of offense for which a waiver would be needed. You need to discuss this with an experienced immigration lawyer for if a waiver is needed, I would strongly suggest that someone prepare that for you.
While this probably will be classified as "one petty offense" and have no impact on your case, you should share the documents with an experienced immigration attorney to make sure.
(213) 394-4554 x0 Mr. Shusterman is a former INS Trial Attorney (1976-82) with over 35 years of immigration experience. His response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.
Contact the court clerk where you entered your plea. Find out what was / is the record in your file respecting the plea. Especially look to see if there was a recorded plea with advisement of rights. Take those documents with you to your immigration attorney.
The above is not intended as legal advice. The response does not constitute the creation of an attorney client relationship as this forum does not provide for a confidential communication.