• After downloading the MIUI theme in .mtz extension on your computer, copy this into your device storage or SD card using a USB cable.
• Go to app drawer on your phone and launch the Themes app.
• Under Theme tab select Offline.
• Here you’ll see the list of all offline themes that comes pre-installed in your smartphone.
• Scroll down and tap on Import.
• Now, access to the destination folder, and tap the .mtz theme file which you have saved to device storage/SD card.
• The theme will be installed and it will be shown along with other offline themes.
• Select the theme and “Apply”.

After this, The new theme will be automatically applied to your device.