Best way for overwriting plugin css with custom one

Unfortunately you will have to write a small plugin for this which is in my opinion, the best and cleanest and safest option. As you have already stated, it is a really bad idea changing any file in a plugin or theme that you are not the author of, and this goes for core files as well

I will tend to not go with a child theme here due to the fact that theme might be changes later and then you will have to do this all over again.

You can do something like this

• Create a folder in your plugins folder and call it what you like

• Create a main plugin file and call in what you want. Open it and add the following at the top of that file

  <?php
  /*
  Plugin Name: NAME WHATEVER YOU LIKE
  */
  

• Create a style.css file in your plugin folder and load all your custom styles in there

• Next, enqueue your custom style.css. You have two options here, either use the $dependency parameter in wp_enqueue_style() or simply add a $priority to your action

Here is an example of the latter

add_action( 'wp_enqueue_scripts', 'my_custom_styles', PHP_INT_MAX );

function my_custom_styles() {
    /* Register our stylesheet. */
    wp_enqueue_style( 'myPluginStylesheet', plugins_url('stylesheet.css', __FILE__) );
}

Your plugin main file should look something like this

<?php
/*
Plugin Name: NAME WHATEVER YOU LIKE
*/

add_action( 'wp_enqueue_scripts', 'my_custom_styles', PHP_INT_MAX );

function my_custom_styles() {
    /* Register our stylesheet. */
    wp_enqueue_style( 'myPluginStylesheet', plugins_url('stylesheet.css', __FILE__) );
}