There are only two preset options (in dark mode): a lowest contrast bluish theme and a higher contrast dark theme.
I prefer lower contrast themes as they are easier on the eyes (text isn't being burned into my retinas), but I dislike Kagi's default bluish theme because I tend to find non-greyscale background colors uncomfortable. I also try to avoid the colour blue on screens at night.
My general preference for dark-mode themes is a dark (but not too dark) grey background, with light grey text. Discord's dark mode colour scheme is an example of this (note that it's very slightly blue, but unnoticeably so).
It would be nice if there were some more preset options for themes. I think some lower contrast grey themes in both light mode and dark mode would be good options regardless of my personal taste since they're neutral.
However I'm sure other people have their own variety of preferences, so I think it would be more scalable to just give some basic, user-friendly settings for background color, text color, accent, etc. in light mode and dark mode.
I don't use the custom CSS feature because it has no documentation and I found it too hard to figure out the weird variable stuff and get the correct identifiers for things. And regardless the custom CSS feature is a non-starter for non-technical users in this regard.
Thanks 🙂