Great to hear you’re using Dim!
Those automatically-changing background colors are a great find, indeed. I’m currently using ⌘+I to switch between my dark and light themes in Terminal.app, but changing the system-wide appearance and having the background update automatically would be nice to support for custom themes.
Dim’s syntax highlighting is consistent on dark and light backgrounds, but the gray colors can be tweaked by changing the bg value. (:set bg=dark on dark backgrounds, :set bg=light on light ones). That will dim the gray line numbers and color columns to use “white” (7) on light themes or “bright black” (8) on dark ones.
I haven’t found a way to automate that yet, because you can’t seem to read the background color value from the current theme. But since we can check the appearance in macOS (defaults read -g AppleInterfaceStyle), it should be possible to switch the bg value based on that. I’ll dive into this a bit more!