CSS changes through Agent Browser UI Extensions

Content

How to I change look and feel of Agent Browser UI?

- Adding CSS files don't work.
- Adding CSS to JS files don't work. Because apparently document object is not accessible in BUI Extension AddIns.

I am trying to change the simplest of things: Adding a Background image to the entire UI.

The entire UI is enclosed within a DIV with id = mainContainer
If I add 'background' attribute to #mainContainer through Inspect Element then it works.
But how do I add CSS properties through BUI Extensions?

Version

Service Cloud 17D

Code Snippet

             ORACLE_SERVICE_CLOUD.extension_loader.load("1").then(function(IExtensionProvider)
	{
        IExtensionProvider.registerUserInterfaceExtension(function(IUserInterfaceContext)
		{
            alert("loaded");
            document.getElementById("mainContainer").style = "background: url('/euf/assets/images/home_page_image.png')";
		}
	);
	}
);