First of all you will need this on the
head section on every
<meta content='width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no' name='viewport'>
Then, in the CSS file you can make a responsive design able to adapt to different screen sizes. For example on your css file you define:
font-family: 'Arial', sans-serif;
In this example we have defined
body as a general element with the same properties for every screen size, but
#top is defined in a way that on desktop will have a top margin of 40 px, and when the screen width is below 800px the top margin is 5px.