In order to enable the iCloud skin on roundcube, you will need to navigate to Settings > Preferences > User Interface. Within this interface you can define the theme you want to use for each device (Desktop, Tablet, Mobile).
Settings interface - under the Interface Skin section you just click on "Classic" and a popup will show up with the iCloud skin being an option.
This is the popup that shows up after you click Classic on the above screenshot. Select iCloud to choose this skin.
Once you click save, your interface will show up like this using the new iCloud theme.