Whiskeycoke,
I'm a little confused about the new format. I was using the old zips and everything seemed to work ok. (theme manager was very slow scrolling)
So I deleted those and tried to download the new mtz files on my phone but it just opened the file in a browser window, weird. Then I used my computer to dl and transfer to my phone. loaded up dark and transparent for the transparent stuff, then wanted to load AOSP icons. It only gave me the option of default or dark and transparent.
Tried a different system style and it changed everything, except the icons. I tried various configurations but could only make icons default or dark and transparent.
So I put the zips back on and all is good. Anyone else have a problem w/the mtz files?
Do be honest I haven't been able to run 12.18 on my phone but loaded it up again to take a look. I did the same thing you did, installed dark and transparent and it worked fine. I then went into theme manager and and clicked on the Icon tab, then clicked on Themes Icon where I saw a couple of themes listed. I then went to the bottom of the Themes Icon page and clicked Load Themes and navigated to the /miui/themes folder and selected the Rounded Icons theme and loaded up those icons. After the reboot I had Dark'n Transparent in the System Style and Text Style boxes and Rounded Icons in the Icon box.
I tried to load up a few more icon themes but none of the other ones would allow me to select them so something must be different in those theme packs. We'll have to dig into that a bit more to see why we can't pick and choose between the different theme selections.