I don't think Firefox Color can do that. At best you could test and set colors using Color, and then export the settings for both as a theme .zip or .xpi files, and then combine the two to a single manifest.json file. Inside the manifest, a "theme" key would be color properties for "normal" theme and "dark_theme" would be for dark-theme. After that you would submit the theme package to get it signed after which it can be installed as a real theme.