If you go to /admin/site_settings/category/all_results?filter=interface_color_selector on your site you should be able to see the setting.
If you switch your system preference in your phone/computer from light to dark (or vice versa), does your discourse interface change to match the system preference or does it remain unchanged (in light or dark mode)?
Given that the core selector is working here and many other sites, there must be a configuration issue somewhere on your end that’s causing the selector to not be shown. Can you share the output of these commands when you run them in your browser console while you’re logged into your account?
(({dark_scheme_id, color_scheme_id}) => { return {dark_scheme_id, color_scheme_id}} )((await (await fetch(`/u/${Discourse.User.current().username}.json`)).json()).user.user_option)
Discourse.SiteSettings.default_dark_mode_color_scheme_id