Louis, here you can find the theming tutorials. You can customize the look and feel in your application by creating your own theme. Basically it's a predefined html container layout which you can style with css.

You can insert your own or third party javascript library for features which aren't available in mendix. So it's very easy to create a simple theme, but creating a complex/advanced theme is also possible.

As long as you have the right people with JS/HTML/CSS3 skills, as Erik said, you can do anything you want. Please look at the customer section of the mendix website for some examples of application built with mendix.