I created a custom logo.img for Mi Mix 2 (chiron) with instructions to create your own one.
This is not a bootanimation, is the logo shown when phone power up.
Tested on Mi Mix 2 (chiron) with xiaomi.eu rom. Dont try on another devices.
I created a theme file (.mtz) with boot animation only. It can be open with any archiver tool like WinRAR and replace the bootanimation.zip file inside with your favorite one.
1. Open Themes App > My account > Themes > Import >...
Mi phone is Mi Mix 2. Installing 7.12.7 TWRP show error "unmount of /system failed" but update finish and seem to work. Just to be sure I wiped all partitions and flash again, this time without errors.