You can override the color scheme with CSS (some detail here: Updating themes and plugins to support automatic dark mode), otherwise you’d need to build a custom plugin to allow you to set a different theme for an admin, as it’s not an existing feature within Discourse.
The idea would be to put the admin CSS in your default theme; unless someone went digging around in your CSS you’d be the only one to see/utilize it.
Another option entirely is to use a browser plugin like stylish to set custom CSS locally.