CSS & Javascript
The very definition of look and feel belongs to css and javascript. There is a lot of power in these two things and a lot of issues you need to be mindful of. CSS effects everything on your page from how something is colored, how images appear, even to where content is placed on the page. Javascript provides websites with the power of action as it allows your users to press buttons, process requests, and even fast foward videos through scripting actions.
If you plan to update your site's CSS be mindful of ATI compliancy issues that you are held responsible for. If you plan to use Javascript to create interactive sections you should always plan to have it disabled by the end user. Your content needs to be accessible even without Javascript being available.