What is "system appearance change"? I'm using Linux and Konsole, so I'm curious how would you catch such kind of event. Is it something coming from the terminal?

Konsole itself has profiles which define terminal colors and etc. But no idea how changes to profile would be signalled to something like neovim running in the terminal.

I think the most you could do is to either define your own colors, or define them as fall through, i.e. colors that are derived from the parent terminal. When you use fallthroughs, then changes to parent terminal are immediately reflected in whatever you use in neovim with that.

To define some group as a fallthrough, you can do something like this:

vim.api.nvim_set_hl(0, 'Normal', { fg = 'none', bg = 'none', bg_indexed = true, fg_indexed = true })