Instead of Custom CSS Theme CSS is loading

It is not clear if you want to just override the styles or replace the file with a new one.

Whatever the case is, below code will help. Just wait for the best answer to your question.

add_action( 'wp_enqueue_scripts', 'add_my_style_css_also', 99 );
function add_my_style_css_also() {  
    wp_enqueue_style( 'my-styles-css', get_stylesheet_directory_uri() . '/style123.css' );  
}

The code will go in your themes functions.php file.

Change /style123.css – to /folder-name/style123.css if your new file is in some folder.

In case you want to remove the parent style sheet, please use the below:

add_action( 'after_setup_theme', 'remove_parent_style' );
   function remove_parent_style() {
   wp_deregister_style( 'styles' ); // or style, whatever is the handle of the sheet
}

You can also try the wp_deregister_style line in the ‘add_my_style_css_also’ function above.