If you want your custom CSS to apply no matter what theme you are using, then you’ll need to use a plugin. WPCode is the best code snippets plugin that allows you to add PHP, CSS, and more to your WordPress website without breaking anything. The first thing you need to do is install and activate the free WPCode plugin. For more details, see our guide on how to install a WordPress plugin.Upon activation, you need to go to Code Snippets » + Add Snippet from the WordPress admin dashboard. Here, hover over the ‘Add Your Custom Code (New Snippet)’ option in the code snippets library and click the ‘Use snippet’ button.