Questions about MyBB Backdoors

Hello! This question might be pretty dumb & annoying for you / However I need to know since I'm using an out-dated theme for my website.

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I've been running an game-server for a long time. I've purchased scripts & gamemode and much more to have more playerbase and more features in it. (Garry's Mod) However, I have an anti-cheat called 'Cake Anti Cheat' to detech hackers and all other backdoor software. I'm not sure If it's the same in MyBB but I have an theme called Duende. However It's pretty out-dated eventhough its for 1.8x

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What I'm trying to ask is; Is there any way to check backdoors in it if it has one? Or can they do something with theme folders? Is it harmless? I just need some detail. Thank you.