WordPress along with its plugins directory has a folder called mu-plugins which when created inside the wp-content directory, will "force" WordPress to load the "must use" plugins from this directory.

A typical fresh install of WordPress has the following inside the plugins:

Lets say just for arguments sake that you would like to set the error reporting level in PHP to display all errors BUT not deprecation warnings.

You can then create the following file in the mu-plugins directory as per below

<?php 
/*
    Plugin Name: PHP - Don't report deprecation warnings
    Description: Don't report deprecation warnings
*/
error_reporting(E_ALL & ~E_DEPRECATED);

Now when you refresh your plugins page you will notice the new link "Must-Use" which will show the plugins WordPress has been told to load.

If you open the Must-Use link you will see the plugins listed which are being force loaded i.e you cannot activate, deactivate or remove (as the name implied, Must Use!)

Now you will not have your log files building up with deprecation notices!

When working with WordPress themes its always a good idea to create a child theme of the parent theme you are using, doing this is a relatively painless exercise. Create a folder matching the parent theme's name appended with the '-child' i.e. if twentytwentyone is the parent then create twentytwentyone-child.

Two files required for a basic child theme are style.css and functions.php

/*
 Theme Name:   Twenty Twenty One Child
 Theme URI:    http://example.com/twenty-twenty-one-child/
 Description:  Twenty Twenty One Child Theme
 Author:       Matthew Hodge
 Author URI:   http://example.com
 Template:     twentytwentyone
 Version:      1.0.0
 License:      GNU General Public License v2 or later
 License URI:  http://www.gnu.org/licenses/gpl-2.0.html
 Tags:         light, dark, two-columns, right-sidebar, responsive-layout, accessibility-ready
 Text Domain:  twentytwentyonechild
*/

add_action( 'wp_enqueue_scripts', 'my_enqueue_function' );
function my_enqueue_function() {
    $parent_handle = 'twenty-twenty-one-style'; 
    $theme = wp_get_theme();
    wp_enqueue_style( $parent_handle, get_template_directory_uri() . '/style.css',  [],  $theme->parent()->get('Version') );
    wp_enqueue_style( 'twenty-twenty-one-child-style', get_stylesheet_uri(), [ $parent_handle ], $theme->get('Version') );
}

Do take note that when enqueueing you need to update the above code sample parent_handle to match that of the parent theme and to reference the correct style (failure to do so may result in the theme being included twice pending how the parent theme was written), as the style.css itself might be named something different for example main.css.

Last tip when dealing with enqueuing files is that the version will get cached by the browser, in order to ensure your changes are taking effect you would have to update the Version declaration inside your style.css (production) or alternatively you could use the time() / filemtime()

wp_enqueue_style( 'twenty-twenty-one-child-style', get_stylesheet_uri(), [ $parent_handle ], time() );
//OR
wp_enqueue_style( 'twenty-twenty-one-child-style', get_stylesheet_uri(), [ $parent_handle ], filemtime( get_stylesheet_directory() . '/style.css' ) );

If using the time() take care as this should never be pushed to production, what would be best is to have your webpack produce the style from your assets increasing the version for you during your build process (if you have this available).