Cyanogenmod has a neat theming feature to change fonts, styles, boot animations and more but older themes might not work so well. I tried a few last night including one called Cosmos CM12.X. When I applied it a dialog popped up saying, “Unfortunately, System UI has stopped” and it wouldn’t go away!

Luckily I’m not the only one and fixing it is a matter of rebooting in safe mode and uninstalling the offending theme.

if it happens again to you reboot in safe mod and uninstall the theme.
you can do this by pressing the power button until the menu shows on then press power off longly then choose ok reboot in safe mod