Maybe it’s just me but I think the Mumble UI is way better than the Discord UI

Is JavaScript a serious language? /s

Joking aside: One of Brendan Eich’s books probably contains something resembling a style guide.

Since you specifically mentioned C# : https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/coding-style/coding-conventions

I’d be surprised if there is a serious language that doesn’t come with at least some semi-official style guide. But usually they are not universally followed and everybody just does their own thing.