Here is how you can add custom code to your app in the simplest way.
1. Open the F: Theme Access: Insert CSS app and click on the Enable and Configure button. Even if you already enabled it before, you will be redirected to the theme customisation page.
2. Now you can see three input boxes: Insert HTML, Insert CSS, and Insert JS.
3. In the Insert HTML section, you can paste your HTML code and also add your CSS data to insert Insert CSS. Here is an example of how I added my code.