You're looking for "Responsive Design" which is very easy in CSS, frameworks like Bootstrap are there for you.
However, in C3, responsive design is... much more manual.
You'll have to engineer code to resize objects accordingly.
In this case, I have chosen a permanent portrait only design. But if you need a head start, something like this:
if( portrait ) {
someObject.width = 100px;
}
if( landscape ) {
someObject.width = 300px;
}