The most likely solution would be to create a new CSS file that will be active only for screen resolutions that a mobile device would have listed somewhere in your <head>. Unless you're going to change the colours depending on the resolution, I prefer to have a master file that is responsible for only displaying the colours, font styles, etc... . This way, the other CSS files are only responsible for adjusting font sizes and positions of elements on the page. For example:
<link rel = "stylesheet" type = "text/css" href = "master.css" />
<link rel = "stylesheet" type = "text/css" href = "mobile.css" media = "screen and (max-width: 700px") />
You may need to adjust the max-width depending on the mobile device.