Hi, if by “legacy mod” you mean the old UI design, you need to set “newmenu” option to 1 in game config. As of 5.60+, that option can no longer be changed from inside the game client.
On windows, your config path would likely be C:/Users/your-windows-username/Saved Games/Toribash/custom.cfg
Miscellaneous options - Forum skin
https://forum.toribash.com/profile.php?do=editoptions
If it still shows up as legacy skin after your apply the changes, restart your browser or add &styleid=16 at the end of that link and open it again.