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The second method requires root access and Xposed framework
This method only works on MIUI 8 and below, to use this method you need root access. as we know to have root access, we must first unlock the smartphone bootloader, especially for the latest smartphones. After that, install the Xposed framework and make sure its running then install this module on the Xposed framework: For MIUI8 and above version or For MIUI7 or below version

The third method is to use Custom Rom (Unlocked Bootloader)
If you buy an Android smartphone on the market today, most of the bootloaders are locked bootloader. Well, you know, Android is an open operating system? Why is the bootloader locked? This happens because the vendor wants its users to keep using the Android OS version that they have designed for their devices. With a locked bootloader, our access to smartphones is very limited. So for flashing the kernel, custom rom, and custom recovery we have to unlock the bootloader first. If we have done this, our access to smartphones is unlimited, we can freely flash custom rom, custom kernel and custom recovery.
This method is the most effective method for all MIUI versions. This method may require Unlocked Bootloader because newer smartphones have to do it in order to install custom recovery like twrp to install custom rom. To use this most effective method, please use a modified MIUI rom such as miuiam, globerom, xiaomieu, miuipro, etc. If you have installed the modified MIUI ROM, you only need to install the theme in the default theme application as usual.

For other methods, please search on Google or in related forums, use the method that you think is the most effective, simplest and safest.