Flexi has not yet been updated for the later moodle version, but if there is sufficient demand I may try to get it looked at over Christmas. However, this is likely to mean a complete rewrite rather than an update so may note be feasible without some sort of funding. Current demand (and already on the to-do list) is for Krystle to be updated for 2.6 and my own wish to create a bootstrap/flexbox powered theme.
Willing to be guided by the community though - perhaps others could respond and let me know if there is sufficient demand for a flexi for Moodle2.5/2.6+. The way settings pages are now it could be neatened up A LOT! but there are also so many themes out there now that have many settings available (if not quite the number that flexi had) while the original flexi was one of the early adopters of settings which made it stand out more at the time.
If demand is there, perhaps my bootstrap/flexbox theme could become the new flexi for 2.6 :D