I am looking for a responsive theme for roundcube (or squirrelmail) that works for all devices well. I've tried mobilecube and it does not work well for narrow screens. Also I get a weird error when trying to put the SQL in for the calendar, something about foreign key and fails to create the events table.
Sadly, I don't think there is anything currently available.
Responsive skins for Roundcube are available here: http://roundcubeskins.net. They're not responsive in the way that they adjust their looks WHILE you resize your browser, but they do adjust their layout if you view them on different devices (phone, tablet, desktop.)
Something similar for Squirrelmail: http://ensigniamail.com