You could look at how Tecosaur does their splash screen, which uses SVGs and adjusts them based on the theme.
https://tecosaur.github.io/emacs-config/config.html#fancy-splash-screen
https://www.reddit.com/r/emacs/comments/fzz4l6/my_fancy_splash_screen/
Or maybe check out how PDF-tools handles midnight mode
Link to release notes.
https://protesilaos.com/codelog/2023-11-12-denote-2-1-0/