In General
Device Detection Module maps a channel from /server/rendering/channelManagement/channels to a Theme variation like /sitedefinition/variations/smartphone/templates...theme.
Notice: these are the same variations available in the Pages Preview App ... they are defined here: info.magnolia.module.devicedetection.DeviceChannelResolver.
Other
In your Theme, you can set media = all for CSS files if using a framework like Bootstrap or Foundation.
To ensure proper rendering and touch zooming, add the viewport meta tag to your <head>
<meta name="viewport" content="width=device-width, initial-scale=1">