The exact way to do this may vary based on the theme you are using, but here’s a simple plugin that hooks into the wp_head
action hook and adds some style to the header:
<?php
/**
* @package PACKAGE_NAME
*/
/*
Plugin Name: Plugin name
Plugin URI: https://plugin-website
Description: Plugin Description
Version: 1.0.0
Author: Plugin Author
Author URI: https://author-website
License: Plugin License
Text Domain: text-domain
*/
add_action( 'wp_head', 'wpse321903_add_styles' );
function wpse321903_add_styles(){ ?>
<style>
.page-title {
color: white;
}
</style><?php
}