Multi-Threaded Java Server
There's not a lot to say about such a simple program, mainly nitpicks:


Member fields that you don't need to expose to the outside should be private, such as the socket in ServerSocketThread.
Whenever you can, make member fields final, for example the socket in ServerSocketThread.
The Socket parameter in the constructor of ServerSocketThread is poorly named accept. It would be better to call it what it is: socket
s for a Scanner is not a great name either. How about scanner ?
It's a bit odd that the server doesn't talk back to the client, just prints stuff on its console.
If the program gets more complex later:


It might be good to move the literal string like "bye" to a constant variable, as it's somewhat special, being the special terminating sequence
Printing stack trace on the console is considered bad practice
Printing pretty much anything to the console is considered bad practice


However, at this point, your program being a toy, it doesn't really matter, just for the record...