All that needs to change is the stylesheet. Both stylesheets are included in the <head> of the document, however only one is to be uncommented at a time.
There are 2 files which must be changed to switch seasons:
app/views/layouts/site.blade.phpapp/views/layouts/secondary.blade.php
In the case of both files, you will find the stylesheet includes located between lines: 12-16
You just have to comment out one season, and uncomment the other.