The code you’d enter is exactly the same as if you were entering it in a page header. In TW we do this through tagged tiddlers ($:/tags/RawMarkup) rather than messing with the actual page.
And of course introducing css is trivial, put your CSS in a tiddler and tagged with $:/tags/Stylesheet and you can check the order here Control Panel > Info > Advanced > Stylesheets.
Of course then you want to apply the css and use it depends on precisely what you want it to do.