If you're using a Windows computer, decompress the downloaded file, and run MIUINewThemeEditor.exe in the root directory to open the editor; while for macOS, you need to run the editor's subdirectory (the folder named like 19.5.31_1559298827. Folder names will vary due to different editor versions).
Run MIUINewThemeEditor.jar to open the editor.
For a MacOS computer, click here to download java environment first.
Due to the security settings of macOS, the first running of MIUINewThemeEditor.jar may not be allowed by the system. In that case, you can right click the mouse to open the editor. Later you can double click the editor to open it.
The editor directory can't contain any special characters (e.g. spaces); it's better not to put it in the Chinese directory.
Attention
When connecting the editor to the phone, you need to turn on USB debugging. Method:
To enable USB debugging, you need to turn on Developer options first. Go to Settings> My device> All specs, and tap the MIUI version continuously (at least 7 times, until you see the prompt: No need, you are already a developers).
To allow USB debugging: Go to Settings > Additional settings > Developer options, and turn on USB debugging. When you connect the phone to the computer for the first time, you will see a pop-up window "Allows USB debugging?" on the phone, select "OK" to establish the USB debugging between the computer and your phone. And there is no need to open it again in the future (note that after optimizing the phone in the Security, the system may turn off the USB debugging mode automatically. If it's turned off, please follow the steps above to enable it again).
If the editor can't apply or open themes, the reasons could be:
If you connect the phone to it for the first time, it may take a while (not because of device crushing) to open or create new themes.
Can't apply themes after adding plugins.
Check if the editor's plugin directory contains the plugins you added, if not, the merging will fail. You can check the plugin_config.xml file in the theme package.
The handwritten manifest file has a syntax error. Two identical attributes are added to a line of codes, for example, two X coordinates are written in a line of codes. Or required spaces are missing between attributes. The above situations will cause the failure of plugin applying.
Other causes:
Check if the phone's USB debugging is turned on: Go to Settings > Developer options > USB debugging (on).
Open the Task manager to check if there are multiple adb processes. Uninstall or turn off other processes that occupied the adb, or click "Fix adb" in the theme editor's menu (available only in a Windows computer).
The phone driver is not installed. You can click here to download MiFlash to install the driver. Installation method: Open miflash and click the Driver button to install it automatically. After installing the driver, you can delete MiFlash.
The above theme package directory (including but not limited to items listed) is mainly used currently. With the deepening of the approach to theme design and the increase in demand, you can also explore for more possibilities of modules. For details, please refer to the official themes.