Hello,

you can use the integrated editor to edit styles in the main css file named main.css.
The template is located in the default joomla templates folder however.

If you need to preview your mobile site on your PC, you can append this string to urls: ?template=responsivizer
In this way you can easily debug for example using Firebug every style on your mobile pages.

If it's not a production site you can also consider to enable the plugin setting to switch always to mobile template, this let you fully navigate and debug the responsive template also on desktop pc.

Best regards

John