@JuliaChang93 It could be that the custom content just isn't made well for anything except the highest graphics settings. That's true of a lot of custom content, and it may be exacerbated in macOS due to how different the graphics drivers are from the ones in Windows.
If you'd like to experiment a bit, you could manually edit a file called graphicsrules.sgr to change some of the internal settings. The way it works is, you pick a value for a certain in-game setting (e.g. sim details on high or medium or whatever), and that corresponds to a number of specific values the game uses to decide how to render objects or lighting at various distances. But by editing graphicsrules, you can manually change any given value on its own without having to change any others, so you could for example change how shadows are rendered without altering any other lighting settings.
Since you know that playing on ultra settings makes the cc look fine but using high settings does not, you could eliminate a number of values as potential causes of the problem, leaving only the ones where the values applied at high and ultra settings are different. This might seem a bit complicated, but I'd be happy to help you go through it, if you'd like to try. Just let me know.