。
She has committed the crime of theft.
Literal
She [topic-は] theft [possessive-の] crime [object-を] has-committed.
窃盗の罪を犯している breaks the compound 窃盗罪 into its two components linked by の: 窃盗 (theft) + の + 罪 (crime). Japanese freely alternates between compound nouns (窃盗罪) and noun-の-noun phrases (窃盗の罪) with little change in meaning, though the compound form tends to be more formal and compact. The の version is slightly more readable and makes the relationship between the components explicit.