Fomantic is a community fork of Semantic-UI, started last year because of the lack of maintainance of SUI. Since then, the situation on SUI side hasn't improved, and basically nothing is merged or updated anymore, while Fomantic has released many changes (not only bugfixes and dependencies updates, but also new features we could use)
It also has a Roadmap and the community looks quite involved!
For all those reasons, I think it's relevant to switch from Semantic UI to Fomantic, especially given the fact this is a drop-in replacement, as you can see in this MR.
I simply had to fix a slight glitch in the player controls after the switch, but I actually think it's because we had custom code to handle an SUI glitch that was fixed on FUI side ;)
Apart that, based on my tests, nothing changed in the UI after the change.