You can add HTML, CSS and JavaScript to your pages.
To do that, click the stripe where you wish to place the code, hover over the first option, and click + (add element).
You’ll see a menu appear with different types of elements to choose from.
On the left of this menu, choose Widget:
In the Widgets menu that appears, select the Media (in background) tab:
Click on the Code tab at the top menu to add your code:
When you’re done editing the code, click SAVE.
Whenever you are working with HTML code, it's always best to first copy your code to a blank web page or use jsfiddle.net. You can then test the code before embedding it, ensuring that it works and does what you want it to do. You can even test with multiple browsers for compatibility.