I think I am following your question.
You can enqueue a stylesheet only for the admin area and then add your styles to that. The following code will enqueue a stylesheet in a “css” folder in the root of your theme.
function my_admin_styles(){
wp_enqueue_style(
'admin_css',
get_stylesheet_directory_uri() . '/css/admin-styles.css', array(), filemtime( get_stylesheet_directory() . '/css/admin-styles.css')
);
}
add_action('admin_enqueue_scripts', 'my_admin_styles');
All you need to do is create a css directory in your themes root, add a file called admin-styles.css and add your styles to that.