How to edit the border-radius on the forums!?

[InVaLiD]

Hello, My name is Joseph, and I am working on a MyBB forum. I have everything (almost) sorted out and this is (almost) the last step to the forum. Here's my problem: I can't figure out how to remove the border-radius (curves) on the forums themselves, I want the forums to be square looking, and I want the forum title background (like My Category's background) to be a different color. Thank you for your time and support.

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