Use a Mobile layout and a Desktop layout to get different style for your pages.
And, as well, use mobile/desktop specific css classes to style your elements.
In addition to Claire, checkout the Mendix documentation and learning paths, there's a lot about styling there as well. In the documentation there are some styling how-to's that make a good starting point. Need more in depth info? Check out the rest of the docs or follow a learning path for styling.
For native styling in Mx8:
For responsive styling, check out this: