It's not natively supported by UOT because it is an actively developed ROM, so the files needed by UOT Kitchen are changed on a weekly basis.
To build a theme for Mirage using UOT Kitchen, use a program like 7-zip to open the Mirage ROM's zip file. Go into the system/app folder and drag the SystemUI.apk file to your desktop. Then go into the system/frameworks folder and drage the frameworks-res.apk file to your desktop. In UOT Kitchen, select to upload files and then select those two files from your desktop.