Linux provides a standard cross-desktop API to query the system color scheme, see https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Settings.html. ashpd provides a Rust library for these portals, see https://bilelmoussaoui.github.io/ashpd/ashpd/desktop/settings/index.html for access to settings.

I wanted to make a pull request but haven't been able to figure out where the theme information comes from on Windows/macOS, and I wanted to clarify whether a new dependency would be okay.