Question: I just restored a SP 2013 Foundation site collection into another farm.
The restore seems ok, but the web front end of the main site is totally missing CSS. 

The subsites are OK: they all are using the standard 2013 layout with different colours.
I performed the backup and restore by `backup-spsite` and `restore-spsite` respectively.

Answer: 
Links might have broken check in the network with chrome or use fiddler
The ultimate solution was in the database size. We were restoring on a MSSQL express a database whose size was beyond the size supported by the Express edition.
We smoothly restored on a MSSQL Standard Edition