public static void AddThemes(this IServiceCollection services, Action<ThemesOptions> setup) {
    services.Configure(setup);
}

public class ThemesOptions {
    public IEnumerable<string> Themes { get; set; }
}

services.AddThemes(options => {
    options.Themes = Configuration.GetSection("Themes").Get<ThemesOptions>().Themes;
});

services.AddThemes(options => {
    options = Configuration.GetSection("Themes").Get<ThemesOptions>();
});

services.AddThemes(options => Configuration.GetSection("Themes"));

public static void AddThemes(this IServiceCollection services, IConfiguration configuration) {
    services.Configure<ThemesOptions>(configuration.GetSection("Themes"));
}

services.AddThemes(Configuration);