Is your theme set up for custom.css?
https://www.pearsonified.com/2007/02/how_to_protect_css_mods_for_any_wordpress_theme.php
If not, then set that up first. Then wrap div with a class or and id around the WordTube shortcode tag in the post code.
Then define the wrap, positioning etc in the custom.css for your new divs.
Last but not least if you are not using Firefox with the Firebug extension you need to. Viewing, editing, customizing CSS is a breeze with that set up.