Version 1.2 is the latest stable version of FluxBB. It is a direct continuation of PunBB (i.e. FluxBB 1.2.18 is a direct update from PunBB 1.2.17). The latest version of 1.2.21, which introduces several security updates.
All PunBB 1.2 modifications and styles should work with FluxBB 1.2.
FluxBB 1.2 is currently developed further to version 1.4, which introduces some new features like UTF-8 support.