Here's my advice: Focus on whether the change actually solves the problem it's trying to solve. Run the code; does it work? What regressions could it cause? Is there any way it could be simpler?
Along the way, you may notice style guide offences, or minor nitpicks. It's good to take note of those things, but only raise those issues once you've addressed the fundamentals.
At the end of the day, confidence comes from understanding. So dig in and understand the context, understand the effects of the change, and give the best feedback you can to help get to the best solution.