this should be done using Visual Studio 2012-2013 or you can hack at it using SharePoint Designer.
I am currently doing the hacking part and believe me if i had the time i would go back and do it the visual studio way.
When adding CSS to any aspx page ensure you use:
If you want to apply CSS styles to a page layout per device channel use the below:
-->
-->
-->
<link rel="stylesheet" type="text/css" href="MyStyleSheet.css" ms-design-css-conversion="no" />
<!--ME:</Publishing:DeviceChannelPanel>-->
<!--CE: End Device Channel Panel Snippet-->
</div><div data-name="DeviceChannelPanel">
<!--CS: Start Device Channel Panel Snippet-->
<!--SPM:<%@Register Tagprefix="Publishing" Namespace="Microsoft.SharePoint.Publishing.WebControls" Assembly="Microsoft.SharePoint.Publishing, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c"%>-->
<!--MS:<Publishing:DeviceChannelPanel runat="server" IncludedChannels="Channel2">-->
<link rel="stylesheet" type="text/css" href="CSS5.css" />
<!--ME:</Publishing:DeviceChannelPanel>-->
<!--CE: End Device Channel Panel Snippet-->
</div>