I have a project done and it is in the following
import tkinter as tk
from tkinter import ttk
import tkinter.messagebox as tmb
from ttkthemes import ThemedSt
The code worked for me fine.. Looks like you haven't successfully imported ttkthemes i.e. it's not properly installed. Therefore, you should do it again. To install ttkthemes (assuming you already have python3 and setup tools):
sudo apt-get install python3-tk
sudo -H pip3 install ttkthemes
This should work perfectly fine.