How can I adjust the css within fusion theme based on the size of the screen? I'm using the Fusion theme for its responsive components but there are some things I want to tweak on the different layouts based on the screen size of the device viewing.

How can I apply style changes to say the phone layout, but not change the desktop and tablet layouts?

As a working example, lets say I wanted to accomplish the following: 

/* Desktop + Tablet */
h1 {
  color: red;
}

/* Phones */
h1 {
  color: blue;
}


Are there selectors I should be using to accomplish this type of fine tuning?
You might find this post on media queries and width attributes useful.