WordPress Administrators can add custom CSS or JavaScript code to the plugin pages from the plugin settings. The code included in these setting fields are only applied to the pages of the corresponding plugin which these setting fields belong to.
To add custom code to the fields, go to PLUGIN-SETTINGS > Settings and click on the Tools tab. You will see Custom CSS and JS fields, add your code and save. You can use these fields to fix plugin or theme related issues without modifying external files.
The code added here will be not affected from any plugin updates. Please note that "PLUGIN-SETTINGS" menu name can change based on the plugin you use.