The way around that is to use media queries.... basically you wrap a copy of the code above in a media query and change the values within it to suit that screen size.
/* Smartphones */
@media (max-width: 480px) { ... }
/* Smartphones to Tablets */
@media (min-width: 481px) and (max-width: 767px) { ... }
/* Tablets */
@media (min-width: 768px) and (max-width: 959px) { ... }
/* Desktop */
@media (min-width: 960px) and (max-width: 1199px) { ... }
/* Large Display */
@media (min-width: 1200px) { ... }
This is how to create a custom CSS compatible with Gantry 4...
Simply create a file called "<TEMPLATENAME>-custom.css" and put this file in the CSS folder of the template (where <TEMPLATENAME> is the name of the template e.g. rt_fracture). GANTRY4 will automatically load this CSS file. If you wish, you can also have browser specific files by appending, for example, "-ie9" making the filename "<TEMPLATENAME>-custom-ie9.css"