Upgrade from 3.0 to 3.1

Is there a clear guide for what is required to upgrade from v3.0 to v3.1? I’ve seen the /docs/UPGRADES/readme.txt file in the 3.1 download; but that appears to be referring to upgrading from versions older than 3.0?

Will my customised CSS created for v3.0 still be valid for 3.1?

Tennant