This occurs because you are deleting the font glyph file that is being displayed on these themes. You could be deleting them manually from the sdcard/ZooperWidget folder, but more than likely you are using an app like CleanMaster that is deleting these automatically.
If this occurs, just delete the theme and reapply it from the theme store. Then, add Themer and Zooper to your CleanMaster ignore list, otherwise this will occur again.Jim Cox commented
Same problem for me... sometimes it will switch to the alphabet after a day sometimes a week, the only way I've found to fix it is to reinstall every time... getting annoying, please fix