There are a number of excellent responses already orbiting around the potential meanings of "safe", but I feel I can add a little here.
I don't have time at the moment to watch the full video, but if all you're generally doing is altering the appearance of GNOME, you shouldn't have too much to worry about. With Timeshift, you're generally also protected from goofing and losing your desktop environment, so it should be just fine.
It's important to remember that flexibility is part of the GNOME standards and general culture (as it is for other desktop environments like KDE Plasma and XFCE). Not everyone works well in the same environment, after all. So altering things like the menu layout is no more dangerous than, say, changing your desktop wallpaper.
The only thing I've seen that is worth commenting on is the "git clone" part. Github is a private website, and it is not a part of your distro in itself; anybody can upload anything to such services. So, when you get to the part about executing shell files ("*.sh"), you want to make absolutely certain that you trust where these files are coming from. Personally I would shy away from that, but it's perfectly plausible that it's innocuous.
Unless we looked at the actual contents of the shell files, it's very difficult to say how safe this is. Politely, if you're very new to Ubuntu, I might suggest playing with the settings yourself and seeing if you can find something that works well for you, before handing control over to an unknown script writer. Even if it's well meaning, there's always the possibility that a dependency mismatch, or other subtle change, is going to prevent things from working out; with the UI tools you don't usually need to worry about that.
If you do proceed, you should most certainly use Timeshift so that you'll be able to reset any damage; and even then, using it isn't without its time and effort cost.
