How to import the css in the plugin admin area?

Just hook your callback into admin_print_styles, i.e.:

add_action( 'admin_print_styles', 'mytheme_add_init' );

Alternately, you could add an is_admin() conditional wrapper inside your callback, and hook into wp_enqueue_scripts:

function mytheme_add_init() {
    if ( is_admin() ) {
        $file_dir=get_bloginfo('template_directory');
        wp_enqueue_style("functions", $file_dir."/scripts/custom.css", false, "1.0", "all");
        wp_enqueue_script("rm_script", $file_dir."/scripts/custom.js", false, "1.0");
    }
}
add_action( 'wp_enqueue_scripts', 'mytheme_add_init' );

But the absolute best approach is to hook into your Theme’s admin page, via admin_print_styles-appearance_page_{pagename}:

add_action( 'admin_print_styles-appearance_page_{pagename}', 'mytheme_add_init', 11 );

This is a custom hook specifically for your appearance page, as defined via your add_theme_page() call.