JavaScript is blocked, php is allowed, it's just obviously not parsed by SMWC so you can only have it in your CSS hosted on your own server, not in your HTML.
For random files, you'll need some very simple php.
For an avatar for example you could just do this, assuming the files are all .png files and in the same directory as the php file:
example.php
<?php
$avatar = array(
'Name1',
'Name2',
[...]
'Namen'
);
header('Location: '.$avatar[mt_rand(0,count($avatar)-1)].'.png' );
?>
That'd always return a random entry from the array, so Name1.png, Name2.png etc.
You can obviously also put other paths in there, but just putting them all in the same directory strikes me as the easiest.
Same can obviously be used for CSS, but it's probably smarter to actually learn some php so you can have some more dynamics than just switching between entire CSS files.
Your layout has been removed.