Question: how to install mtz theme on miui 9?
Answer: Download the MTZ file of MIUI Theme you wish to install on your phone. ... 
Download the full Mi Themer APK file from here.
Locate the APK file that you have just downloaded and open it to install it. ... 
Now open the Mi Themer app. ... 
Next, tap the “Create New Theme” menu on the top right-hand corner.