Wordpress Code EditorThe Wordpress Code Editor
There are only a few easy tasks left to unlock the editors: Find by: define('DISALLOW_FILE_EDIT', ); modify truth to false: define('DISALLOW_FILE_EDIT', ); store your files and resubmit. You can now use the Appearance Editor on the Appearance page: Your first chance is that someone might have obtained administrative privileges for your website.
If this happens and you have activated your editor, it can use it to insert any code you like. WordPress already provides all the support you need to edit and abuse your website, so you don't need FTP or any other permission. And the second is that it's really simple to make changes in a way that will most likely go wrong if there is an upgrade for that topic or plug-in.
For the most part, it is the topic you want to customize, and there are several ways to do this. When it is just css, then you should use a plug-in (e.g. SimpleCustomCSS) that has a style editor that allows you to modify the style independent of the design. So, if the design is refreshed or you are changing the design, your customized style sheet is still there.
You can also create a children topic. It is an intermediate way of overriding parts of a topic, which is not always so simple. Find out more about WordPress Codex. Getting Started with WordPressThis is how to get started with your new website: fundamentals, contents, topics, safety and more.